You could always drop a 2004R into it - you can build them to handle all the horse power you want, plus they are identical in dimensions to the TH350. Driveshafts slide right in with no mods - same length, same yoke. Gives you a lock up converter, overdrive and no real hassle to install.
There is a visual drawback (not much of one), in that the bellhousing carries a dual bolt pattern - BOP and Chevy. Remember, though , that these were used behind Monte SS's and Grand Nationals back in the day - use the right HD parts and configure them correctly, and they'll hold up behind a lot of HP engines. You'll need to source the Chevy dipstick and tube to mate the bolt pattern to a Chevy block, otherwise the tube bolts to the BOP bolt pattern.
One more good thing - find a local junkyard that has a lot of late 80's GM vehicles, and you've got a better than average chance of finding good core trannies for like $25.00, like I did. Bought several of them. Or, a lot of trans builders offer them as built/ready to go performance trans for about the same money as a 700.
Just my opinion -