Dana 61 Tech- Can it be Geared, what are the advantages
Tech:
Most of the rears are the big spindles and can run 35 spline shafts without being bored, not all check them to be sure before getting to involved! Most of the 61's we have seen come with 3.07 gears, in order to run the 3.07 gears and fit the monster pinion head in there dana changed the offset of the pinion by changing the housing not the carrier. Dana 61's did however come w/ 3.55, 3.73 and 4.10 gears. The best advatage to these, they are cheap b/c the Q of whether or not they can be geared is a bit mystical. And the good news is they can be regeared!
Gearing:
This is how you get one to work. There was a a dana 60 spacer made by Reider that allowed you to run the lower ratio gear ie 4.56 and up. I believe this spacer was 5/8 thick. It required the 4.56 and up dana 60 carrier and the ring gear spacer to get the offset needed. This said spacer seems to be just a legend now and has been discontinued. However their still is the Mr. gasket dana 60 spacer (5/16 thickness) that just about every drivetrain company sells. This spacer is designed to allow you to run the 4.56 and up gears on a 4.10 and down carrier for a regular 60. This spacer does come into play but need not be confused w/ the legendary Rieder spacer, they are of different thickness.
Alas there is a dana/spicer Dana 61- 3.33 and up carrier part# 706400x. This carrier is 2 3/4 tall. Where as the dana 60 4.56 and up carrier part# 706041x is 2 7/16 tall. So what combination do you need to run which gear ratio? If you have a dana 61 that has 3.07 gears in it and you want to run 3.33-4.30 gears, you will need to run the dana 61 3.33 and up carrier part# 706400x or the dana 60 4.56 and up carrier with a 5/16 ring gear spacer. This is popular for people who are trying to regear a 3.07 dodge cummins and the like front 61 that has a non overdrive auto, If you switch to a manual or to an oerdrive auto transmission, you will likely want some lower gearing that is how you get it.
If you have a dana 61 with 3.33 or higher gear ratio all you need is a 5/16 ring gear spacer to run 4.56's and up.
If you have a dana 61 with 3.07 gears and you want to go to 4.56 or numerically higher gear ratio you will need to purchase a dana 61 3.33 and up carrier part# 706400x and a 5/16 ring gear spacer to achieve the desired offset. Or have a 5/8 ring gear spacer custom machined. This is popular for the guys trying to score a cheap front or rear 60's and gear it for offroad use.

Thick Gears, Carrier changes
No thick cut gears are available for the Dana 60 regular rotation axle ie rear dana 60 axle.
There are however thick cut and thin cut gears available for the high pinion reverse rotaion 60's. In order to run 4.56 gears or numerically higher a ring gear spacer or a carrier change to a 4.56 and up carrier is required for a regular rotation 60.

The information in that ECGS link is annoyingly incorrect. According to a post by Carl Jantz, the pinion offset for the D61 is 15/16" vs 1/2" for a D60. In other words, the pinion sits 7/16" farther away from the carrier flange on a D61 vs a D61. I am doing a D61 front right now (got it cheap, which is the big benefit ), and I checked the offset for myself. It was hard to get an accurate measurement, but I got .425" which jives with Carl's claim of .437". Everything on Dana axles uses common fractional increments, so I assume .437" (7/16") is indeed correct. ECGS claims 5/16" (.312") or 5/8" (.625"), neither of which are close.

A D61 with the 3.07 gearset uses the exact same part # carrier as a D60 4.10-down carrier (706040X). D61's with 3.54 and 4.10s use the same gearsets as a D60 (706033-1X and 706033-3X respectively) but with a special D61 only carrier that moves the ring gear 7/16" closer to the pinion (706400X).

So, to regear a D61 to "D60" gears, you have a few options. To do so without spacers, you can use the 706400X carrier to run standard 4.10-down gearsets and THICK 4.56-up gearsets. In order to use a standard 4.56-up gearset with the 706400X carrier you need the readily available .160" spacer made by Mr. Gasket. Since the pilot surface on the carrier is substantially more than .160" thick your ring gear will still be properly located on the hub.

If you want to run a D60 carrier (a locker for example), you MUST use a ring gear spacer. Here you have two choices: a 4.10-down carrier and a 4.56-up carrier. If you use a given ring gear with its normally matched carrier like you would in a D60, then you need a 7/16" (.437") thick spacer. In order to use a 4.10 or numerically lower gearset with a 4.56-up carrier you need a .277" thick spacer (.437"-.160"), while using a 4.56 or higher gearset with a 4.10-down carrier will require a .597" thick spacer (.437"+.160"). Using a thick 4.56-down gearset with a 4.56-down carrier will require a .277" thick spacer (.437"-.160"). I am using the .160" difference between .410-down and .456-up carriers  from the thickness of the Mr. Gasket spacer. I have never actually measured and confirmed that value. The only measurement I am sure of is the .437" difference between the D61 and D60.

Any of those spacers (.277", .437", .597") will be thicker than the ring gear pilot surface on the carrier, so the spacer itself will need to pilot on the carrier and then have a concentric pilot surface for the ring gear. Reider discontinued their spacer and as far as I know NOBODY else makes spacers in any of these thicknesses (except for the .437" thick one I am making for myself :grinpimp:), so you would have to have a local machine shop make you one. Since the pilot ring on the spacer needs to be stepped inwards you will also almost certainly need to do some minor turning of the carrier for clearance (this is the case on mine - it isn't much and is easy to to though). Finally, you will need bolts of the proper length. Luckily, ARP started making generic 1/2" fine thread bolts in 1/4" length increments last year, and combined with their 1/8" thick precision ground hardened washers (if necessary) gives you a selection of high quality 180,000psi fasteners in effectively 1/8" increments. In my case, to run 4.10's on a 4.10-down carrier with a .437" thick spacer I will need 1-3/4" long bolts with a hardened washer under each one to get the same thread engagement as the stock bolts with no spacer. As a bonus the fasteners are 30,000 psi stronger than the stock grade 8 units, and the extra 9/16" of non-engaged bolt shank will give more consistent preload.

Finally, some people are squeamish about running ring gear spacers. As long as the ring gear is properly piloted, it is just as strong as running with no spacer. The bolts are not, AND SHOULD NEVER BE, loaded in shear. All torque is transmitted through friction between the tightly clamped ring gear and carrier flange. With a spacer, there are two interfaces, but each is as tightly clamped as a single interface - the clamping force is NOT divided between the two as some people think. A single 180,000PSI 1/2" ARP bolt develops 16,391lbs of force when torqued to it's 75% yield point. Since there are 12 bolts attaching a D60 ring gear, that is a total clamping force of  196,692lbs or about 98 tons. That ring gear ain't going anywhere

If a guy doesn't mind running an open carrier or a lunchbox locker, the 706400x carrier is the easiest no-machining method to make it work. 4.10-down work as-is, 4.56-up with either thick gears or the commonly available .160" spacer. Only hard part might be finding a carrier if the axle doesn't come with one. The main reason I am machining a spacer is because I fully intend to run an E-Locker or ARB in the future, so I don't want to waste the money on a 706400X carrier to avoid machining a spacer that I will end up needing anyway.

I don't mind if you repost what I wrote. What is RCC anyway?

Cool, thanks for posting it!  I'm glad I gave credit where it was due so I didn't look like an ass, lol.