Seat belt bolts often suck to remove--they are usually #50 Torx
head bolts or have thin hex heads that twist off easy..they put some killer locktite on them usually too..
.I've had the nut welded in the floor twist out with the floor still attached often when I'd try removing them at the junkyard..
We often resorted to using a pipe wrench or vise grips on them after snapping off multiple Torx bits or rounding off the thin hex head..
Heat is a big help,but most junkyards wont allow any propane torches past the gate..