if you're gonna weld up the firewall holes, I suggest to get a step drill and some of these "unholes". the step drill will make a round hole, as opposed to a regular fluted drill bit, and you simply drill the hole to fit the patch piece. that way everything fits better. use a magnet to hold the patch in until it is tacked. the magnet usually distorts the welding electrical field so it will sputter, thats why just use it to tack then get rid of it. these are from Canada so the USD will be pretty good. they gotta be shipped though so....with covid....
another thing I have done is use a regular,non plated, nail head for the patch, if you have a small hole, then once it's in place cut the nail shank off and smooth it out. some guys will use electrical punch outs but they are painted and plated sometimes so they sputter and make a porous weld usually. some machine shops will have plugs from their punch press they sell cheap.
https://welderseries.com/Unholes-p50202282
https://garage.eastwood.com/project-...-unused-holes/