If you have a TH350 and it's good and you're going to use it, then I'd go that route. If you're going to have it rebuilt, then I'd go with the TH400. Craigslist has these parts fairly cheap for BOP transmissions. I looked up BOP turbo 400 in St Louis and found the ad below. The guy wants $190 each and has 5 to choose from. If you're not in a hurry they come up pretty often.
https://stlouis.craigslist.org/pts/d...014111696.html