yarg.
My bellhousing does have a center hole slightly larger than the typical gm ones. Mine is 5 1/8. I was planning on welding an insert in it to make it fit my 4.85" trans. bearing retainer. I assumed that regardless of the center hole, the bearing would still be centered. After all, the middle is still the middle.
I guess not though.
It's such a pain finder a bell with the right size center hole for less than 150 bucks.