Swedester, I don't know alot about what you're asking but I can tell you my experience. I recently replaced the 216 in my 1946 truck with a motor out of a 1939 car. Worked just fine, but had to swap out flywheels because they're different, and mount to the crankshaft differently. The 46 had 6 bolts, (and keepers) and 3 or 4 dowels on the end of the crank. The 39 crank has no dowels, and only 4 bolts. I believe they changed this aspect around 1940.
I also had to modify my exhaust as the oil distributors on the drivers side of the block, are slightly different, and not in the same place.
I might be wrong, but I don't know that there is any difference between the 41 and the 51 motor. However, I'm sure someone will point out the errors I've overlooked.
Good Luck.
