What are my options for bolting a different trans up to a slant 6 bellhousing?

The transmission you have is probably the A-903. It has synchro on 2 & 3 gears. Ratios vary depending on whether is was behind a 170 or 225.

Engine 1st 2nd 3rd
170 3.22:1 1.84:1 1.00:1
225 2.95:1 1.83:1 1.00:1

I can't imagine one of these behind a Max Wedge car, or for that matter behind the 361 2bbl in a Chrysler Newport. Had one in a 66 Dart with a V-8. It was a PITA to have to pull to a stop to get to first gear. Yeah, I could double clutch, but that was a PITA, too.