Block out of square

I feel it’s “extremely unlikely” the out of square condition at the back of your block has anything to do with the block machining that was done as part of the rebuild....... unless they machined the bellhousing flange itself out of square.

It would cost a ton of time and $$$ to try and correct all the stack up problems that would follow that big of a screw up.
You’d just throw the block away and start over.

Like I said earlier......what timing set are you going to use in that situation?

If you’re comfortable shimming the bell housing......I say go for it.