How to fix the stuck in free-spin bolt?

i was working on my car and needed to remove the two front bolts to the cross member so i can make room for the front mount to come out. when putting them back in, i had trouble because it was rusty. I was doing the forward reverse on the bolt to work it up. when doing this the nut that it was screwing into broke its weld. (its a nut welded to the frame.) now i can't get to the nut because it is encased and the only way to get to it would be to cut a hole in the frame. this part of the frame has the radiator bolted to it. would cutting into this weaken this part of the car? is there any other way to fix this?