I understand what you want to do and will chip in with this. I disagree about using a TH350 behind a big block. Now, if you've spent money to beef it up, then it will work fine. But let me ask, did you ever see a TH350 behind a big block from the factory? I don't think so and there's a reason for that, because a big block will frag a stock 350. There are fellows who use beefed 350's at the drag strip, but it's because they're trying to find a couple of tenths due to the lighter rotating components as compared to a TH400.
Big block, TH400, 9" rear (measure a late 70's Bronco) for cheap, bulletproof fun. 049 or 781 heads from racing junk, ebay, craigslist, etc. Google "049 heads for sale".