I plan on doing the same on my 48 Chevy pickup. I have welded up holes and cut and welded bumpers and had them replated a number of times so it isn't that big of a deal. My first thought is to just cut the head off, stick the bolt in the hole and weld it up and grind it smooth. I think, that is what I will be doing. HOWEVER, you can't tighten up the nut a bunch. You are likely to get some kind of "ghost line" or "dimple" or something in the face of the bumper. I am thinking to use a jam or lock nut so it doesn't have to be cranked down on.
But I have also seen where guys would put a "shim" type plate with a slotted hole in it on the back of the bumper. Then slide a regular old "stove bolt" bumper bolt into the slot and put the bumper on. The problem with this is, it will hold the bumper out a little further than it was originally. Unless you modify the irons of course.
I think I am just going to weld the bolt into the hole.
Brian