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

Let's see if I am reading the post right. About the only tranny that will bolt to the '64 bellhousing is another 3 speed. The bolt pattern is not right for a 4 speed. You have 2 choices for transmissions, the one you have or the '68-'74 which has synchronizers on low gear. The tranny you have has a crash low gear. These 3 speeds are tough as nails and have been around almost as long as Chrysler. They came behind Max Wedge 413s so I doubt a blown slant will hurt one. The shift levers are in the side case. They are not a "toploader". To switch one over to floor shift you will need a 3 speed floor shifter and cut a honkin' hole in the floor for shifter rod clearance. There are ways to use a 4 speed or more but not with what you are getting from the yard.