First - nice truck! Second - great project, and completely doable
. Third, what's yer name?
Okay. Yah there are several threads about diesel swaps, but I think you'll find taking the time to thoroughly read those threads and taking lots of notes will very likely provide all the tech you'll need, including things to be aware of specifically important to you, things to avoid, etc. Grab a couple brews and take the time to fully capture what you're wanting to do.
That said, being a 3/4-ton 2wd truck your swap will be much less 'involved' than most. Typically, most swaps involve a complete donor truck and that is just to have most of the parts up front and not have to go find all the pieces. But, being a 3/4-ton truck I think you already have plenty of 'frame', and if you already have a D70 rear axle then you really just need the 6BT engine and 2wd manual transmission. That said, be sure to have the original D70 inspected and/or even re-set to ensure the ring and pinion are 'perfect' - maybe even go so far as to have a brand new quality set installed, and drop in a limited slip while it's open.
I'm not 100% certain which motor mounts you'd use, but I think a 6BT will 'bolt up' to your existing 2wd gasser engine crossmember, but the transmission likely will not and you'll have to drill the holes and such to install the crossmember with the diesel transmission.
Your big core gasser radiator will likely be plenty for your intended use, and like has been mentioned you'll just need to make sure everything clears (fan, fan clutch, etc).
I would recommend an intercooled engine/trans even if you don't use the OE intercooler. You can go 'up and over' the valve covers with the intake, or, you could do some voodoo and use an aftermarket aluminum radiator and the OE intercooler and electric fans, or, if this is not going off-road and will be a nice highway cruiser, you could run the OE radiator and have a custom shop fabricate or buy an intercooler that mounts wherever it can fit (possibly under the radiator) and modify the intake piping to match.
Dash. For sure you will need the two standard 'diesel' gauges - pyrometer (especially important if you are considering a non-intercooled chassis) and boost, and those can go 'anywhere' - just so long as you can keep an eye on them when driving. I'm near certain the G360 or NV4500s or NV5600s can use a mechanical speedometer output, which you will need to drive your speedo.
Your factory 70's dash will interface with the 6BT the same as any gasser engine - water temp, oil pressure, volts, etc - all same-same just in the 'Cummins' ports vs the gasser ports.
Suspension. IMPO, being a 3/4-ton already...I actually don't think you'll have to do 'anything', at least I wouldn't do anything to anything until the truck is rolling with all the weight in the bed (camper, gear, etc). I predict you will actually have an outstanding ride. That said, if you want a better ride in back, I'd do the mods to relocate your front/rear leaf spring mounts and install a set of 'low-lift' 63" leaf springs (i.e. Pro Comp 13211), and then if necessary mount a set of air bags to control the weight. The ride will be much less harsh.
Performance. Turbo, exhaust, fuel. If I were building your truck I'd for sure run a 'better' turbo - like a new HX-35 with...ooohhhh...say a 14cm wastegated collar set at about 35 psi. You'll also definitely want...no, need
...a 4" exhaust and downpipe. Assuming you don't want your truck to make you deaf, I'd recommend installing a 4" baffled and chambered muffler, and while I haven't yet 'heard' it I just installed pn# 7081Q from Heartthrob Exhaust - near the bottom of the page - Heartthrob Exhaust - Diesel Kits
. While the 4" mufflers from Dynomax and Magnaflow look cool they are not good at reducing sound and still allow lots of 'drone' in the cab.
If you don't have a D70 out back - if it's a D60 (or even if it is a D70), I'd highly recommend replacing it with a newer 01-10 GM 14b full-float rear axle from any 2500 truck of the era, which will include factory disc brakes and incorporated e-brake - all off-the shelf, and bulletproof (make sure it isn't the newer (2011-up?) 8-lug with the just-slightly-bigger metric bolt pattern).
Interior. You will want every available application of sound insulation available, including a dash/firewall mat, hood liner, more firewall insulation, inner fender liners, and lastly, more hood and firewall insulation. 1st Gen Cummins' make a LOT of rattlerattlerattle...which is just the nature of the beast - they're awesome beasts but they are noisy beasts. I am actually working on making my own oil pan and valve cover covers/blankets, just to contain the maximum amount of 'rattle' that I can.
Next, please do check out my 'De-computerize your Dodge' thread as there is a lot of applicable info considering your plans. You don't have any computers in your 75, and you don't need any for your 6BT or trans to 'run', which is excellent. Assuming you use a 1st Gen intercooled engine you'll want to do all the 'easy' mods, especially the 366 spring. You can get plenty of power with the OE injectors, but bump up the fuel a bit with the full-fuel screw and then adjust the other things to clean up the smoke. I'd delete the OE WIF sensor with the conversion nipple and run the upgraded fuel filter. Definitely, definitely
kill the KDP.
If you're looking for hardware (downpipe, turbo's etc) I'd recommend contacting Mark Wilson at Dieseltuff.com (in Canada, eh). He's a friend of mine and a long-time diesel performance guru.
Okay, that should be pretty good to get you started. Post up your plans, questions, progress, and for sure, pics!