I have a 65 Barracuda with a slant 6 - three on the tree want some transmission/engine advice

Back around 1990, I had a 65 Valiant 100, 170 slant with three on the tree. I swapped in a mildly modified 225, also an all synchro 3-speed floor shift from a 70-something Duster. I'm pretty sure I used the original 65 bellhousing. I know I used the 65 flywheel, although I did have to have the center hole bored out to fit the later 225 crank. Also had to cut the floor for linkage clearance, but the trans fit fine. I cut the floor shift hump out of a Duster (maybe the same on the trans came out of, can't remember) and cobbled it to the early A floor pan. Later found an original cast iron early slant six four speed bellhousing, and swapped in a four speed tranny. Now that was a super fun car to drive!

If you can find the parts, either the all synchro three-speed floor shift or the four speed would, I think, be an easier swap than converting to automatic. Also more fun.