Herb,
It's easy to replace a TH350 with a 200 4R because they are the same length and the driveshaft does not have to be altered in any way. The 200 4R is more compact than the TH 400 making it easy to fit it where the TH 350 fit. When I did that conversion on my roadster street rod the changes I had to make were moving the rear mount rearward about 6 inches, getting a correct shifter and getting a TV cable and geometry correcter from Bow Tie Overdrives who also provided the transmission.
If you do some research on the 200 4R you'll find out what the enthusiast magazines have to say about its ability to handle various power levels.
The 0.67 OD ratio would be a nice match with your 3.90 rear end.
Good luck and have fun!
