My minor is nearing the end of a mammoth restoration. Today I fitted the carpets and that went very well. What didn't go so well was trying to bolt the seats back in. On the passenger side (original floor) 3 out of 4 bolts were fine, one wouldn't fit no matter what I tried. On the driver's side (new floor) none of the new bolts fitted into the new captive nuts. Why?!

So what to do now?
Replace all 4 captive plates with new ones - not easy as I'd need to weld from underneath and I don't have facilities to roll the car, or drill through the captive nuts and use new nyloc nuts welded in place of old ones. Or is there anothe solution?
Has anyone else had this problem? Grateful for views!