"If you cut and splice in more metal it is no longer an engineered structure."
And that's the kind of FUD which should be ignored.
After all, insurance companies ignore that every time they ensure a vehicle which is modded or has a custom fabricated body or section.
For a variety of reasons a welded join will rarely be stronger than the original material, but will frequently fail in the heat affected zone near the weld . . .
Under the inside skin of my bus, there are welds and spot welds all over the ribs. Clearly, Thomas is not that concerned.
Skoolie.com sells inserts up to 24", and the vertical extent of a rib above the chair rail is about 4' (or more, 4' is a minimum). Raising the roof by 2' reduces the overall factor of safety by about a factor of 1.5, make it 1.65 for good measure) . . .
. . . and that neglects the strength added by reskinning window deleted sections. The cumulative strength of the skin in tension resists the rib's deformation in bending (the skin itself resists bending in no way at all, but the ribs can't bend without the rivets pulling on the stretching skin). So it's not really as bad as 1.65.
The thing is full custom fabricated bodies go on the road every day, they are not "engineered" and insurance doesn't bat an eye.
Which is why your comment may sound correct to the uneducated but is FUD.