[Lynn Eades]

Yes you can use the Mercury crank in the ford block. The Merc. has a 4.0" throw, so the piston height is different. If you have one you are lucky. Ford throw is 3 3/4"
The water holes at the centre of the blocks changed shape at times and is a way of identifying the year of production.
Alistair, I have heard (not substantiated) that the "increase in the H.p. was to do more with the increased octane rating of the available fuel, than any particular improvement of any engine component.
The merc crank has a circle cast into the front counterweight, and has a larger gallery plug in the front big end drilling ( Merc=5/8" plug. Ford= 3/8" plug)
There is a lot of info available on the web.