Install Steps

1. Check fitment and mark. Place the filler panel onto the outside of the driver and passenger side firewall and check fitment. With a permanent marker, trace a reference line along the edge of the filler onto the existing firewall for both driver and passenger. Using a cutoff wheel or electric grinder, cut inboard roughly ½″ along the marked reference line — masking tape can help guide your spacing.
2. Separate the braces. There are 2 braces running along the inside of the firewall at an angle on either side of the engine tunnel — separate the braces from the firewall sheet metal.
3. Drill out the spot welds. Use a 5/16″ drill bit to drill out the spot welds along the outside of the firewall that attach the sheet metal to the braces. At this point the old firewall should be separated from the cab.
4. Clean the lip. Leaving a ½″ lip around the existing firewall gives the new fillers a slight edge to rest on for welding. Clean the perimeter with a grinding disk or abrasive tool to remove old paint — you’ll need clean metal for welding.
5. Position and tack weld. Place the filler panel onto the opening until it lines up nicely with the top and outside edge, then tack weld it into place. Slight trimming may be needed in the middle of the firewall where the two fillers meet — roughly 1″ of material is left for cab diversity.
6. Finish welding. If you’re happy with the placement, tack weld around the perimeter at 1″ intervals to minimize warping. Once welded, grind down your welds and prep for body work.

Further Reading

A couple of great articles on replacing a Chevy truck firewall: