Recently I mentioned to a local gunsmith I have a Martini Cadet that's been poorly drilled for scope mounts in multiple places on the barrel and the holes were such that I couldn't just put a screw in them and file it flush.  I said I was considering just having the holes welded up or going ahead and having it re-barreled.

If I understood him correctly, he said he would mix a powdered metal with black JB Weld, fill the holes and then properly drill for a new mount.

An unrelated conversation we had a few days later plus part of another I overheard while standing at the counter waiting to look at something convinced me he wasn't the guy I was looking for.

My question is, what metal would he likely have used?  I don't recall if he said aluminum or steel?  Or would either be acceptable?

The best result will be threaded filler rods, and if the surface is hogged out or messed up around the surface I just drill the holes the next size larger, and tap them again. 
I never use screws as they wont fit tight enough to not show a gap around the hole. I use rod and thread it so it wont bottom out in the hole, and instead it will stop as the threads end. Then cut them off a few thousandths above the barrel, and peen them down before filing them to match the surface. The peening will expand the plug tightly into the surface, and when they're draw filed they disappear. Very tough to find them after the rust bluing is done.
I do the same with dovetail fillers. Make them very tight, then peen each end before draw filing to fit.