I don't know what your floor pans looked like, or mine for that matter because I'm out of town at school, but what about just jacking the frame rails extensions into position and welding them in? Would the floorpans pop into the vehicle into a concave? Could the extensions just be clearanced easier than the floor pan? Just wondering? I want to do extensions in my 510 too.
Great in depth write ups too. I've used the POR 15 gas tank sealer kit. It worked great.