Will a 77 Charger 360 fit in a 65 Valiant Slant Six Painlessly?

on A body, yes.

the B&T drive shaft continued thru 65. the shifter moved to the column with mechanical (cable) linkage or the floor with cables. splined output came in 66
Not sure about early A bodies, but B and C cars received the splined output / slip yoke in 65. In those cars, 65 is a one year only trans, being slip yoke and cable shift.