If they are the bolts which hold the rack to the crossmember I'd suggest drilling and tapping out to a larger thread and using a slightly larger bolt. There should be enough material in the threaded nuts to cope.
I guess they'd be M8 x 1.25 (top of head) and so there may be something slightly bigger in imperial that'd do.
Or cut the old rooted threaded parts out and weld in a few nice new nuts and use new bolts.