I second that TH400, even if you want an overdrive later. THEN I'd put on a gear vendor, which is the route I went. Very happy with my decision, vs the also popular 200r4's OD tranny's.

But my setup will outlive anything I can throw at it, which was part of my decision.

FYI, I purchased my 400 through Oldani, quite happy.

 

If you want the most economical suggestion I would opt for a TH-400. A TH-350 would live behind a mild motor, even a BBC. But, the durability of the TH-400 will win out especially if you up the power level later on. Or, you can look to an overdrive...