I machined mine myself to match ( it came chevy style ) the id of the retainer opening . Mine us verrrry tight . +~ a couple thousandths
If your bh is not square , theres no way the collar is slipping on and through . The collar is about an inch
Thick . Thats enough to know if youre square and concentric .
If you really want to second guess square a flat
Edge across the bh and collar to verify any high spots .

If youre not square , then you may need to break
Out with the the correct measuring tools to shim and tgen check with the browel tool , but ..
Youre more likely to be using dowels than shims .

I wouldnt want to shim for squareness ..

My clutch shop in rochester here said that
Many bellhousings have been decked perfectly flat by machine shops. But not much to shave off before causing other problems.
If youre block is not square , then either you should reblueprint
Or i guess shim ..