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

You will need to consider what vintage radiator you are going to run and set up the front of the engine for that...

The early bodies had the lower radiator outlet on the driver's side, the later bodies (70 and later) had the lower radiator outlet on the passenger side... You will want to use the period correct timing chain and water pump set-up to match the vintage radiator that you plan to use... They moved the timing marks 90° when they moved the lower radiator hose outlet on the water pumps... The early style used the cast iron water pump, the later style used the aluminum water pump with the outlets on the opposite sides from one style to the other...

It's not hard to do, you just need to know which way to go to make things easier to match up...