What gauge is the firewall on a '50 3100?
Should be 18 or 19 gauge. 19 gauge isn't readily available as sheet metal stock, but 18 gauge is, slightly thicker.
If you're doing any other patching, just buy a larger piece and cut your plugs out of that.
Use a '0' tip on your OA torch and buy some real welding rod 1/16 inch diameter. People have used coat hangers or bailing wire, but there's no QC on those.
[on edit] Just measured mine. 0.052" including primer on both sides. 18 gauge is 0.048"
Also, some of the holes are oval, not round, so you'd have to cut your own patches anyway.
