I am restoring a 77 Can Am, and I have found some serious rust in my floor pans. Luckily I have found a source for some replacement sections, but I have found that they will not extend far enough up into the firewall and tranny tunnel. My question is what will be the best way to fill those gaps? should I cut out the sections for the pans and then fix the rust that extends further into the adjacent panels prior to welding in the new pans? or is there a better way?