4 speed to auto crank?

I have a 360 that I plan on mating a 4 speed 833 up to, problem is it came from a truck with an automatic. I read a little doing some searching around that boards that the crankshaft needs machine work in order to fit the input shaft from the trans. But I'm curious what exactly it is that needs to be done and why the crank can't accept a 4 speed?

I also read, if I understand it correctly, that intalling a '94 or newer pilot bearing into the crank will solve the problem without having to do any machine work on the crank. Is that true and if so are there any drawbacks to it?