'64 Valiant SLANT 6 -rookie question (Chevy guy!).

Assuming the engines are both 1964, they will interchange (with minor mods at the most) but they're not the "same". The manual trans cars have a crank with a finished hole for the pilot bushing where the automatics usually didn't. The manual trans engine will bolt onto the auto but the auto crank engine would need to be drilled larger for the bushing or a bushing would have to be cut down to the unfinished hole diameter to fit the manual trans. The distributor had a different advance curve and the carb was jetted slightly different also but that usually poses no problem. Swap them if they do. Keep in mind that they made both a 170 and a 225 engine those years.
Why not fix them both? Wagons are starting to get popular again.....
Thanks, Mark