I was only off by about 1/32", but it was enough to do it over. I am using a wire feed welder (Lincoln 135 Plus) and flux cored solder. I'm not sure what type of steel I have, but I can't change it now!
I bought it from my local steel supplier, so I assume it would be a common, cheaper type. The metal I'm using is only 1/8" thick (the hole is 1/4") so penetration isn't a problem.
How do I preheat the metal to 500 degrees F? I don't have a 10' x 4' x 2' oven.
Can I just use a propane torch for a minute or two? Do I need to get the whole part to that or just the local area? The first hole was in a piece that weighed 350 pounds or so. I figure it would take more than a propane torch to get that to 500 degrees.