Front cab mounts

[WhiteCST]

I could really use some insight on these bolts, and I really hope that this isn’t operator error. I’m trying to remove the cab on my 68 and everything is ready to go except for the front cab mount bolts. I assume they’re carriage bolts as I don’t see any obvious way to reach the top of the bolts and wouldn’t you know it, the nuts just spin on both sides. I’m going to give it a shot with an impact tomorrow, but before I do, has anyone else had this problem? I have to replace the cab floor, so if I cut these bolts, can I replace them with the new floor? I’m sure this is an amateur question but I don’t want to create more work for myself.