[Evenprime]

I own two automatic 76 W100s and the first has a 318 with a A727 transmission which is visibly marked on the side of the transmission.

My other one does not have such markings. Its mated to a 400ci BB
Not sure if it is a 3speed or a 4 speed

Not sure where to look - the numbers I have found are

3898856  M5274  14something?
374328A-8  where the A-8 could be something else - hard to tell?

 

[dodge82273]

the 518 4 speed will have a second electric connector , it is driver side , behind and above the one you see in the photo ( that is reverse lights and neutral safety) . if I had to guess I'd say 3 speed 727 is what you have there .

 

[Elwenil]

There are no big block overdrive transmissions and there are no smaller transmissions used with the big blocks than the 727, so pretty much if you have a big block then you almost certainly have a 727.  The A518, aka 46RH, was not an option until 1990.  There are adaptors to mate a A518/46RH overdrive transmission to a big block, but it would be pretty obvious if an adaptor was used.

 

[dodge82273]

although your trans does not look original to the truck , I see no adapter ...

 

[DODGEBOYS]

yup 727 is the ONLY tranny used behind a BIG block