Good point Northern, I never even thought of mentioning that. Absolutely, NEVER weld or drill the top or bottom flanges of the rails
Yes, there is less flexing and twisting behind the housings, however, seldom have I ever seen a truck with a skirt or hitch welded on that hadn't had cracks welded up over time.
As for welding at the rear of the rails which is often seen, is it not proper anymore, it was accepted practice for many years but it is something you would never see a high end rig up utilize even years back, for example, Lennox, which still is top of the heap in the patch. Everything was always bolted.
Often guys like to weld hitches and such on to a truck because it is easy and quick, well quick and easy isn't always best. And the rules have changed. headache racks on new rig ups require an engineers approval, as do walking decks, ramps, subframes and the skirt (rear hitch) as well. Anything that has to do with pulling or carrying the load and protecting the driver. Those laws come in around 2000 give or take a year or so. When doing a CVIP it is the responsibility of the mechanic to check for certification number prior to checking off the boxes on the forms. Older stuff was grandfathered in. Yes there are a lot of mechanics that don't bother to check.
Of course farm trucks even though commercial get away with a lot of the rules, but safety is always forefront and should not be jeopardized just to save time or costs.