Ever swapped a 49 Merc crank into a 50 Merc and find it had more stroke?

[Cecil-Indiana]

Gentlemen I've got a mystery here. I rebuilt my 50 Merc engine and found it had a bad rear main leak. When I took it back down I found the rear seal dia. was scored bad. I have a nice 49 engine so I swapped the cranks and the pistons were hitting the Offy head. I think I've just now solved the problem by using 2 head gaskets. Both cranks were dusted .010. Anyone ever run into this situation? Keep in mind all I did was swapped cranks and then I had a bad knock which turned out to be the domed piston hitting the head.