Are you 100% sure the block is the original Mercruiser block from the factory and not a re-man? Is the metal serial # tag riveted on the stbd rear side of the block? (it would be if it were the original block) If please post up the serial # and I will call Mercruiser at work as soon as you let me know and ask them what the deal was. The new motors from Mercruiser and all remans That I have EVER seen are 2 bolt mains.... EVEN the new stroked 377 MPI CI motor is a 2 bolt main. and all it is is a 350 mpi with a 3.75' forged crank. and clearanced rods and oil pan area.
Thanks,
Robby