Metal prep

I just fabricated floor pans and trunk for a '56 Plymouth from 18 gauge sheet metal, but there is no way I would have done it had there been decent floor pans available. In my case, you couldn't even find decent used parts, but you can buy either new pans or decent used parts for any A-body.

The surface rust is no problem. Get one of those black foam-looking, paint stripping discs for your drill, or better yet, an angle grinder. They remove rust or paint in seconds, and won't scratch the metal like sandpaper. Much easier, much faster, and better results, and they work well when cleaning old metal for welding.

Don't bother priming the panels before installing. Any paint will make getting a good weld nearly impossible. Wait until they are installed before using a rust reformer and painting.