I finally have one of my cars on the road, my 396 71' Chevelle. Having driven it now a few times i have come to hate the t400 with its wheel spinning shift kit fitted.
I would like to convert it to an overdrive auto, but just need some advice on which auto to choose.  I don't want to have to fit an adapter plate if possible & no computer either or at least a simple wiring job if a computer is required.
I'm not affraid to buy a whole car to get the right trans, so if anyone can suggest what model of local car if any would suit, i am happy to source one & do the install myself.
i can then fit the T400 into my Camaro for some drag work

Im no expert but I have a Turbo 700 attached to my 454 in my Suburban and that came from a Chevelle that had it behind a big block! So I reakon a T700!

TH700R4 is the way to go. No electrics other than a simple "lock up" kit should you chose to run a locking converter....TCI and others make the kit for this.
The box is bolt on to the engine, and I'd say you'd be able to buy kits from Bow-Tie Overdrives for the crossmember and all the little parts to finish the conversion off.

You can easily source one out of a local VN-VP Commodore. Don't go to later model than that because it will require a computer. The VN-VP's run the T700R with a manual kickdown cable which will have to be connected to your throttle linkages. Other than that it as reasonably simple hook up with only the crossmember requires slight modification due to mount location. It uses a different yoke which is same as Trimatic so your tailshaft will need to be cut and shut also. As long as your engine does not make more than 450 foot pounds of torque it will survive for a long time.

Cheers, Chris. 

Like Chris said, VN/VP commodore is a perfect donor. The T700 I had in my 68 Chevelle is now in Dirtyrats suburban.  It is pretty much a bolt in job. My chassis had extra holes for cross member location,yours should be the same. I had to move my cross member back 2 holes but I had a T350. I think the T700 mounts in the same place as the T400. There are throttle linkage adaptors available for various carbies for the TV cable so that you can get the cable geometry correct. You can also buy a shifter detent from OPGI that will convert your stock shifter to work on an OD trans.
Wiring is really easy, and you only have to add wiring if you want to run a lock up converter.
I think I still have the detent kit for a horse shoe (console) shifter and a modified drive shaft if your interested & decide to go with a T700.

Thanks to all who replied, does the t700 need to be out of a V8 or will a V6 one have the right bell housing?

Has to be a V8......there is no such thing as a V6 T700.