Do you have the original flywheel? If so, it can be used on a later motor if you have the center hole machined for the diameter of the later crank flange.
Bellhousing? The early 4-speed bellhousings are rare. They are cast iron. If you have the original, that would be great. Later ones are aluminum and usually - if not always - are for an overdrive 4-speed and have a much larger opening for the larger OD trans front bearing. They would need a sleeve to fit an early non OD transmission. Also may need clutch linkage mods to use in a 65 Barracuda.
As for which size slant six, the 225 would have the most torque and, all other things being equal, more horsepower. I used to have a 65 Valiant sedan. After finding an early cast iron slant six 4-speed bellhousing, I swapped the original 170/3-on-the-tree for an early seventies 225 and a 65 4-speed with 3.09 low gear. Milled the head .100, hotter cam, four barrel carb and 2.25 inch single exhaust,. With Addco sway bars, 340 torsion bars and rear springs, and 14x6 slot mags and radial tires, it was one of the most fun driving cars I've ever had. Sold it to a good friend when I moved from Atlanta to Tallahassee.