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

The 64 was a Push Button trans so it is cable operated, you will need those cables and brackets. There are 3,4 and even 5 speeds you can hook up to a slant 6. The factory had 3 and 4 speeds. At slantsix.org there are conversion directions to the 5 speed as well as a pictorial on all the different trans available. ( Articles section)
Hope this gives you enough to chew on.
Frank