I’m sure Zippy can answer who made the Mega Blocks.
I don’t think it had anything to do with World, but I could be wrong.

There are fundamental differences in the two blocks.
The full time top end oiling, the extra oil pump feed, and the cross bolted front cap of the World block are some of them.

Some of the easy ways to tell a “Mega Block” from a “World” block are the extra pump feed fitting on the front lower drivers side of the block, and the cross bolted front cap that are on the World block.
They also have core plugs on the from of the block, and the cylinders are bulged out around the cylinders for extra room for coolant.

The only reason I brought it up at all is that a “Mega Block” and a “World” block are not the same externally.
So it’s possible one of those blocks has more of a starter fitment issue than the other........ but I’m not clear on which block the OP is working with.