I don't think I've ever dropped a belly when at least one of the weldnuts didn't break. Even on near new rigs, they break off and spin.
Some people make such an easy thing so complicated - some even cut a big hole in the frame to access it.
Since I don't like to fiddle around, I simply cut the spinning bolt's heads off with a 4 1/2 grinder, torch, or plasma cutter.
The pan will drop off, leaving the studs hanging down. Whack those studs with a hammer - they'll fly up inside the frame somewhere, never to be seen again. Now the hole is clear. If the weldnut was spinning, the weldnut goes flying with the bolt, so the inside is flat again.
When putting the pan back on select a bolt the right length and size you want.
Weld - or JB Quick Weld - length of weld rod or coat hanger to the heads of those bolts.
Simply sneak those bolts in though another hole in the frame somewhere till it protrudes down the hole you want it in. There are big enough holes both in front and in back of the belly pan area. (Tips - mark the rod the length you'll be going in, sneak it in till you see it through the hole, then pull it through the hole with a small magnet.)
When you tighten it, the bolt head will jam against the remaining weldnut pieces inside so it won't turn. An impact works great. If you ever take it off again the rod you put on is usually enough to stop it from spinning.
Simple, easy, fast, and costs near nothing.