Just about every machine tool I've had to replace a motor on in my past, I've had to make adjustments on the motor mount to accept the new motor. The latest one is my 15" Sheldon lathe. I don't recall the frame size it was, the newer one is a 184T frame. Didn't even come close. The old set of holes were on a larger spacing and the new frame size, there was not enough room for new holes. So I cut a couple pieces of flat bar that would span the old mount and provide room for new tapped holes for the new motor mount.
For conversion from a 66 frame to 56 frame, I'm sure you will have to do the same or something similar. As for the shaft size, it maybe the same and it may not. Something tells me the shaft size may be the same, just the base dimensions are different. May get lucky and the shaft height may be the same too. Without more information, I couldn't tell you.
BTW- you won't find a new motor with a 66 frame size today. I suggest, making the adjustments for the 56 frame size motor so that way if someone after you gets the lathe and needs to change out the motor, they will thank you.
Ken