I recently faced the same decision...700R4 is attractive becuase of over drive and a certain level of reliability. If you're going to do a lot of towing then the 700R4 would'nt be a great choice. The TH 350 is a proven performance type trans but is limited in driveability and gas mileage. My decision was the TH400...it's as close to "bullet-proof" as you can find...with certain modern rebuild upgrades that are now available it is far and away the best street/performance trans available.
I've mated my 350/330HP with a rebuilt TH400(about $600. no core)and a 3.42 rear in a '52 Suburban... along with an Edelbrock 600cfm carb, I recently averaged over 22mpg on a 400 mile, relatively high speed trip (VA to NJ).
The trans performs well, runs cool, is a good towing trans and has an excellent reputation for reliability.
