I'd go with a TH350 or better yet a TH400 or for the best of all worlds go with a 200R tha has overdrive for good freeway cruising and almost as tough as the TH400. But a little more work, and with any of the above you may need to re size your drive shaft. BUT you will be much happier with a bigger, badder tranny in the long run! I have had a TH400 behind a 400, 421 and now a 455 with no problems at all!

Here is an on line article that will give you all the details step by step on the benifits of a 200R (about as strong as a TH400) but a 5 speed! And step by step on installation etc... http://www.atlantabuick.com/NFSArticle.htm

Good luck and let us know how it works out!
Brett 67 Drop Top 455!