The TH700-R4 is popular among the small block crowd. It can "take" a bit of power. There are several vendors that offer beefed-up versions.
For the higher torque and power of bigger engines, we prefer TH200-4R. They "live" behind the big Pontiacs where the TH700-R4 doesn't, so they can take ANYTHING a small block can dish out.
TH400 is the toughest of all of them. It also "uses" more power to "drive" it than other transmissions (larger internal parts). TH400 offers a unique shifting capability, however, making it the most popular 3-speed auto trans.
TH350 will be fine behind a small block, too.