First I’ll say that I’m far from an expert on all this .
Does the TA in question have PS pump and bracket, and alternator and bracket? If so then find a later-model 400 and the accessories should bolt up. If not then a later model 400 out of a Trans Am / Formula / Firebird, with all the brackets and accessories that would be ideal (at least alt and PS – you youngsters can live with out AC right?). That often isn’t too easy to do locally (not sure where you are). I don’t know if a 400 out of a Bonneville has the same brackets/accessories as a TA 400 or not. Don’t know where you live but you need to check emissions requirements and inspection requirements in your state. Here in TX the emissions requirements on very old cars are not too bad, but ask guys from CA or AZ – emission controls / state inspection can really be a hassle sometimes when changing engines.
The TH350 is standard equipment on all later 2nd gen TA’s with auto trans and it is fine for anything short of a “seriously” built engine. As stated, you have to get a “BOP” bolt pattern (Buick Olds Pontiac) on the bellhousing. Chevy is different and will not bolt to Pontiac and vice –versa. Some TH-350’s have both patterns (Chev and BOP) aka “dual pattern”.
Lastly, people do in fact drop Chevy motors in TA’s all the time, but what is no big deal for someone who’s been doing this stuff for a while may be exceedingly difficult for someone who is just getting started and has limited tools and no stash of bolts, parts, etc. Don’t forget that time has an assignable dollar value. For what you are about to spend in money and time resurrecting “Frankenstein” you may well be able to buy a running car or one that is more-or-less complete and can be made to run much more easily.
