Ok Tom, I know the part you're talking about. Those buckets use a 2 piece plastic molding to trim out the bottom parts of the seats that probably wasn't found on a bench seat. I think I would try just removing the outer trim piece that'* held in place by a few phillips head screws and see if it functions ok then and looks acceptable. If so that sure beats relocating your welds and/or drilling new holes in the floor pan especially if it has any rust issues. I certainly wouldn't try relocating the seat belt anchor bolt outboard for clearance unless a hefty reinforcing plate was welded underneath the floor pan. Oh, another thought, you might ditch the plastic cover for the seat belt reel and see if you get clearance that way, that might not look as bad as the seat with the outer lower molding removed.
1971 Chevelle SS
2004 Bonne SLE