I just replaced the firewall in my 37 Ford with one from Direct Sheetmetal and they said to grind off the flange and weld the edge after the spot welds have been rewelded. http://directsheetmetal.com/pages/typeinstall.html

they provided self tapping sheetmetal screws to screw into every other spot welding hole to hold the firewall in place while you're doing the new spot welds. As long as the body was bolted to the frame during the firewall removal, I doubt that it shifted and the new firewall should pull everything back in place.

I'm assuming that the installation procedure wouldn't be much different between a 34 & 37 but I could be wrong.

Randy