You'll be able to see the edge of the captive nut once you get the mount down out of the way enough. You just want to tack it enough that it won't move when you take the bolt the rest of the way.

Then to weld it back on, strip the paint away from the area, and have a single sided spot welder used around the edge of the hole while holding the nut against the unibody with the nut, and viola, the nut will be held in place, and you won't have to cut a hole in the frame to do it.

Of course, if you don't have a mig or stick welder and a single sided spot welder, you'll need to get to a place that does.