First thing to do is make sure you have a block that will accept the turbo bell-housing (trimatic & turbo blocks are different).
Turbo 400s were stronger out of the box than 350s, and the later 400s had the lock-up torque converter which some love and some hate.
Turbo 700s are 4 speed and more economical. They came in different strengths for different engines, and you'll need one from before they changed from hydraulic to electronic lock-up (or you can buy a kit to emulate the computer and frig the electronic ones).
That's about all I know. As with most questions like this you'll get people who love powerglides, swear by trimatics, or be really one-eyed about a particular Turbo series.
I'd go a rebuilt Turbo 700, but I'd have a good talk to a trans specialist first to make sure I got exactly what I wanted. Not something I'd buy off eBay.
