Don't forget the custom water lines, the dizzy is also another problem,
the headers might have to be modified a little to fit the 20v.
Most headers are designed for a 16v(unless they made specific for the 20v DansToy makes a very nice header for the 20v). If you shop around you can go as low as $700(full swap tranny, engine, etc).
About the wiring I know someone who can do all that for you and then send it by mail to you, he's on Club4ag. Don't let all that discourage you though, just takes a little time, buying parts here and there. Yeah the tranny you have would be fine. It also depends if your car is an SR5 and you want to swap a EFI 20v(more work to convert it).
If you do have a SR5 what you can do is buy some of the things a GTS has(
diff, 4ag engine, engine mounts, etc) and swap them over. Basically convert a 4AG into a carb side draft setup(similar to Hibino's and Immamura's old carb ae86s). It would help if you put a little work in the 16v if you go with carbs(hi-comp pistions, stronger rods, Cams, etc or just buy a Hi-comp redtop engine). There are way more things I just did more of a summary sort of thing. Any questions just as me on the PM or go to
www.Club4ag.com (check out the tech reference page, it will help you out a lot)