All slants will physically bolt in and mount to a slant six auto transmission or slant six manual bell housing.
A couple of things to watch for:
If your 61 is an automatic transmission the torque converter will have the small snout or centering nub on it. In ‘68 Mopar went to a larger diameter torque converter centering nub and receiving register on the crankshaft. Mounting a 68 and newer engine to an early small nub torque converter is not an issue as an adapter ring that fits into the crankshaft register is available.
If you have a manual transmission, Chrysler made crankshafts with the crank drilled to depth to receive the transmission pilot shaft but not honed to size for the bushing. The work around for that with the large crank register is a late model pilot bushing that mounts into the crank register itself.
Not a fitment issue but there was also a change to hydraulic lifters in the mid 70’s.
Or have you considered having the existing engine rebuilt? There are slant motors available, mainly from folks swapping in a v8, but even the very best of those motors are 40+ years old and really are nothing more than running core engines.
