I got interested in this when I'd sent an engineering friend of some photos of a g/s frame that had been reinforced and he'd chuckle and say they should read Tony's book etc. usually because what they'd done made things no better and probably worse. If I were going to do this I'd have a someone who I had absolutely no doubt understood this sort of chassis design and fabrication. At worst I'd mimic what HPN has done. There are some good photos around.
and some great close ups of a HPN frame here.