I'm thinking of buying a '91 D150 with the 3.9 and the 3-speed auto. It's dirt cheap. It also has a massive transmission fluid leak, or at least that's what it looks like with the big brown puddle under where it's parked for sale. And brown would also be - Not Good.

Besides the obvious part about unbolting the transmission from the engine, removing the starter, disconnecting the driveshaft, taking off the mounting bolts and crossmember that would always come with a transmission R&R, what else needs to be changed to use say, an A500 in place of the A998 that's probably in there?

Linkage?
Cooler Line placement, size, length?
Driveshaft yoke?
Torque converter?
Flexplate?
Mounts and crossmember?

Would the driveshaft need to be changed as well, as in, is the A500 longer than the A998?

I know nothing about the Dodge trucks other than what I find on Wikipedia and Allpar. I know that if I want to put a 4-speed AOD in place of the 3-speed FMX, I have to make a lot of changes. Bellhousing needs to be cut off and replaced with a JW Ultra Bell (to fit the 351M which is BB Ford), trans mount needs to be flipped around, crossmember might need shaved a bit, linkage should fit but I won't be able to reach 1st lockout, cooler lines are a different size so I gotta use adapters, driveshaft will be an inch too long and yoke is different, flexplate will fit but torque converter's different, et cetera.

I'm hoping doing this on a truck will be easier. Also, since I'll likely be sourcing the A500 from a junkyard, does it have to come only from a V6 truck 89-93, or will newer years work and up to what year? What about a trans from a 318? A Dakota? A B-van? I'd love to know what will work and what won't. My searches on car-part.com tell me 89-91 transmissions interchange through all models of those years (Dakota-Ram Truck-Ram Van), and 92-93 transmissions interchange through all models of those years. So I'm curious to know what changed in those years and why, say, a 94-95 3.9 A500/42RH transmission wouldn't work on a '91 3.9?

edit 1: Found one other thread from Izzy08 pertaining to this and seems I'll need to swap engine computers as well. I can handle that. Will just pull it from a V6/overdrive auto truck, 89-91. Should work. Also mentioned driveshaft would need to be shorter, I'll try to get the one off whatever truck I get the computer from. I didn't look at this truck carefully, was in a hurry, might be 92-93 and have the Magnum V6, but with no fender badging and the big chrome grille I was assuming 1991 only. Could just be a repaint with no badges put back on.