Sorry, what you have is a bellhousing that is correct for a '81-'86 truck application, not a car. There was no ball stud linkage made that will work in a car without cutting and welding.
What you should have is the #3743643 bellhousing for '76-'80 overdrive F bodies and trucks, which has the correct pivot bracket.
If you insist on trying to use your current bellhousing, then as you mentioned cutting & welding the fork or grinding and drilling the bellhousing for the proper pivot are your only options. Personally, between those two options, I'd drill & mount the correct pivot and use the known correct fork. Just be sure to make very careful measurements so the bracket is in the exactly correct spot. You may have to grind away part or all of the stud pivot boss to gain full movement of the new arm/bracket, but that will render the bellhousing junk if it doesn't work out.