Anchor Nut spins

I am in the process of installing a lift so when it came time to install the transfer case drop kit I have one stock bolt that just free spins. I was able to stick my finger in a hole in the frame next to the problem bolt, and sure enough the anchor nut is spinning inside with the bolt. I have a few ideas on how to address this, but it is always better to have an outsiders perspective on things. How would you get the bolt out, and how would you install the new bolt since the anchor nut is no longer attached? There is one small hole by the problem, otherwise it is enclosed in the frame. Thanks.

I was lucky to still have a welded nut, but I was ready to make a hole in the floor just above the nut....

Get to it through the floor. I had to do mine this way. I used an 1.5" hole saw. You might have to remove either seat depending on which side has the offending nut.

I just screwed a small plate over the hole when I was done.

Thanks for the input. This is probably what I will end up doing.