A friend put a drivetrain from a cougar eliminator (mercury BOSS351) into a 71 Maverick which doesn't have anymore space than your 65. It ran great but required 3 extensions, 2 ujoints, and jacking up the motor slightly to change the spark plugs. He eventually redid the shock towers which made it managable.
If you are looking at a 351C and want to make sure it isn't a 351M, look at the bellhousing bolt pattern. 351C is the same as a 302, 351M is the same as a 429/460. Basically look at the two bolt holes at the top of the block. If you can put your thumb on one and your little finger on the other easily (about 4-5inches) it's a Cleveland. If you have to really stretch your hand (about 9 inches), it's an M.