floorpans glue in or weld in

I wouldn't worry about the strength of the panel bonding adhesives if the proper one is used, as many have said most new cars are put together this way and the bond strength exceeds the strength of the metal in most cases. Its also used quite a bit in the Aerospace industry, so think on that if you've taken a flight on a new airplane, as most have composite panels and use panel bonding adhesives to attach them.

My only concern would be that the panels weren't originally intended to be bonded, same with the replacement panels, so you may not have the surface area or fit that you would want to in order to use the panel bonding method as its intended.

That and I actually think spot welding them back in would probably be the faster, more straightforward way to go if you don't already have experience using the panel bonding adhesive.