Go with the 700R4, its a relativily easy swap.
You will need a bracket that mounts near the carb for the TV-Cable.
If you can locate a "caprice" style tailshaft on the 700R4, you can use
the auto-xmember in the TH400 position. You may need a longer speedometer cable.
If you have a short bed, go to a one-piece drive shaft.
The 4L80e requires a seperate computer to control, unless you have unlimeted funds, stick with a 700r4. Just keep in mind, if the original 700r4 comes from a "car" it may have shorter splines inside the trans compared to a "truck" 700r4. It really makes no difference, but you need to know this if you have driveshaft made. I have done this and its worth the effort. Even if you dont find a "caprice" tailshaft, you can still use the stock crossmember....but, you will need to drill holes apprx 1-3/4" back from the TH-350 holes, the upper ones can be difficult at best (with a cab in place) bottom ones arent too bad. Check out
www.bowtiedrives.com -or-
www.700r4.com same website, different name, has a lot of good info. The 700r4 is a good trans if its built ok -AND- the end user sets up the TV cable correctly. Another note, the 700r4 uses a lock-up converter, be sure and add the circuitry to deal with this on a NON-computer controlled vehicle.