Originally Posted by vwnate1

The floor boards proper are steel, mild steel that's fairly easy to work with and weld .

Glued to them however, are the thick padded fiber sound deadening pads ~ you'll need to scrape those loose / up before you weld or you'll never get any seal in the floor .

I can't hear the dash noise but clearly you have an exhaust leak and I bet it's under the hood and drawing into the HVAC .

Look for fluffy dry black soot ~ that'll be the leak .

I bet no one told you you'd be a hands on Automotive Detective when you took on this project .