There is another engineering issue with typical air bag mounts not mentioned above. Some of these mounts cantilever out from the C channel frame (the newer ones not as much). This puts a twisting stress into the frame channel. If you look at the spring attachments, they are also cantilevered out so what's the difference? Usually there are cross members placed right behind or very nearly behind the spring mounts. These cross members resolve the twisting stress produced by the spring mounts. There are no cross members where the aftermarket bag mounts attach.
The Ford engineers aren't stupid (well generally speaking anyway :)), but they cannot engineer the frame to allow for anything some aftermarket supplier might think up. The problem is that the aftermarket suppliers very often do very little engineering at all.