Mike,
I have to warn you about your electric fan/toggle switch configuration. I did the same on an old Datsun years ago. The problem is that once in a while you will forget to flip the switch and inadvertently over heat your engine. I never blew up the old Datsun engine, but I did make it spew like a geyser a couple of times. If you go that route, research a method to incorporate a water temperature switch.
Also, it sounds as though the previous owner has figured out the water pump length issue as both the 57 and 61 engines have the long water pump. You may just need to swap over those items from the dead 57 engine.
Paul
