[dsraven]

if welding the holes, look into welders series for their "unholes". buy a pack of same sized ones and then drill the holes in the firewall to fit that size. use a step drill for a nice clean and uniformly round hole that the plug will fit into. then you can use a magnet on the back side to keep the "unhole" in the right spot and flush with the firewall while you spot it in place. leave a little space all around the plug for the weld to penetrate well. a magnet will distort the welding so just spot them in then remove the magnet and use a flap disc to smooth the tack out. then weld a 1/4 of the perimeter and planish, while still smoking hot, with a hammer and dolly. this will help the area cool and not distort. go to the next hole on the other side of the firewall and do the same. move around a bunch to keep the heat down in any one spot. I have heard of guys using electrical box plugs but they are usually plated or painted so that involves a bunch more work. if you have a fab shop close by you could ask if they have any punch outs from their punch press but welders series are pretty cheap. Canadian money so the USD buys more for less right now.
I have heard of guys using autobody panel adhesive to glue a new panel over the old one as well.
if cutting the old one out ensure to brace the cab in an X configuration in the area so things don't move/shift/sag when the firewall is out or the new one is welded back in.