I am doing a frame off restoration on my bronco and when I got to the frame, well lets just say I had a lot of welding to do. When you start to check for holes, start looking underneath the rear shock mounts. Usually some pretty corrosive stuff can get up here due to off-roading and road salt (what happened to my eb). After you have located the holes make sure you cut the area you are going to replace pretty far out from the perimeter of the hole, because although it is not rusted through the rust still may have thinned the metal in the general area out. When you cut the hole out, try cutting at a slight tilt inward, this will allow you to bang and shape the piece of metal you are going to fill the hole with without it falling into the frame constantly.   Next after you have finished cutting out your hole, cut out the new piece of metal. This may be hard to find and buy, especially if you don't want to buy in bulk, but I was lucky to have some left over square tubing from a previous project. Its important to remember that when you trace out the piece you are going to cut, cut on the OUTSIDE of the lines you trace / measure (remember it's easy to take off, but not as easy to put back on).   Now that you have the right shape, start to weld it in. After you weld grind it down till it is ALL the way flush w/ the frame and the extra piece of metal. After grinding look very closely at where you think more or less the seam is. If you see a very small crack you must weld again and then regrind until there is no more crack. If you don't do this the integrity of your frame may be at stake!!!   Tech article by Dan (bane)