It depends on what you're doing. Traditionally, you'd probably be okay with a TH350 if you're just dropping a crate motor in, and that's probably what the truck came with. That or a manual. The TH400 is extremely strong but overkill, in my opinion, especially if you don't plan to throw performance parts on or in the engine. The 700R4 would allow the truck to run on the highway at more modernish automobile RPMs, and you'd get better fuel economy. The only caveat here is that if you're just gonna go out and buy one of them, you're best off to get one that's newer than 1987 and the newest one you can find. They have electronically controlled torque converter lockup so I'm guessing to put them in a truck that didn't come with lockup, you'd have to get a lockup controller, and it'd be a good idea to beef the transmission up some because a lot of people consider them mediocre in durability and longevity positively correlated with the strength of the powerplant. So the bottom line is a pre-lockup TH350 to just throw in and be done with it (which should sufficiently handle your engine), a TH400 if you plan to build the engine or upgrade it down the road, or a 700R4 for good highway performance while understanding that newer units are made much better than the earlier ones, and that if you were going to go with that choice, you'd have to establish a way for the TCC lockup to work.
