Read the post and I had a thought that I haven't heard anyone mention. I've attached a pic that vaguely illustrates the welding technique I am considering. Not sure if it's a good idea, but it seems to eliminate what would appear to be a byproduct of welding around a single cross section of a subsection (frame-rail). The idea is to cut the new section at angles that will be welded along the 'rails' in such a way that no two welds are adjacent to each other, therefore the old metal does not have a single cross-section weld that creates an 'overheat' weak-spot.

Any questions?