2 bolt main wedge blocks were fully water jacketed HD stock replacements, and megablocks were not. The same tooling was used to cast the outside (including the lifter valley) on both, but on the inside they're completely different.
The ones marketed as "megablock" were siamese.
Note Hemi blocks were made with the same water/siamese variation, but all the Hemis had crossbolted mains regardless.
Now if you check it and it's siamese, THEN it's a mistake. More than likely it's just a regular 2 bolt water block.
