The way I'd do that is remove one bolt at a time, put a backer inside the cab and plug weld the support to the firewall. You'll never get them out again, but that's probably not a big concern. BTW, the bolts are carriage head, and are secured to the support, so you could just take one nut off, grind the bolt flush or a little below the surface, then weld the bolt stub to the firewall. Repeat for the other bolts. Pretty easy, and no need to crawl under the dash to do any welding.
