As long as you keep everything from flywheel, starter, bellhousing back you won't have any trouble. I'm running a 1961 six in my old 46 and even kept the foot starter, no sweat. I'd even keep the original style water pump, possible by using an adaptor plate, available on ebay, that allows the old pump to mount to the late block. This is important as you want your fan up on the radiator vs down to low by shortening the shaft etc.
