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

IMO, the easiest and cheapest way to go from your pitiful 903 junk box to a 904 automatic is to find someone parting out a '65 slant 6 automatic and getting the transmission, cables, and steering column. that would be a straight bolt-in....no driveshaft change needed. For a bolt-in slip yoke trans and upgraded yoke/driveshaft, get a '66-7 transmission, and a '66 column with shift linkage. For a floor shift manual trans setup, the '64-5 833's are not too pricey, but, that will require a full shifter with linkage, a slant 6 4 speed bell housing, and a floor cut. You would also need to make sure the crank is drilled for a stock pilot bushing, or, get a conversion bearing for it. There again, the existing driveshaft can be reused. Using a 230 3 speed or later 833 4 speed would need the same conversion, along with the correct slip yoke and driveshaft along with a different bell housing.