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

Most of the efforts to put anything other than an A833 behind a slant six have used the bellhousing that came with the transmission. There's a factory adapter (now pretty hard to find) that puts a V8 transmission behind a slant six (IIRC it's the big block pattern) and there are slant six to Chevy bolt pattern adapters. If that thought makes you wince, there were transmissions that bolt up to a SBC that GM didn't build in-house. :)