Slow but steady progress. Ended up doing the majority of the welding on the inside flange and just tacked on the exterior. Should make for less grinding. Still, turns out welding on a body with paint on the exterior gets pretty smoky. Was very careful to never get the patch panel any warmer than slightly warm to the touch.
Mocked up flare placement and made the cuts. The double shears didn't like making sharp turns so I cleaned it up with the jig saw afterwards. If you have the RV conversion windows like me, this will be an extra step in the process as you don't have the factory window frames or internal bracing to use as a guide.
Next step before body filler is adding in some reinforcement. The shuttle conversion company really took a big piece out of the interior rib - pic with the donor piece shows that even it doesn't 100% cover of their hole up. So I'll either add that donor piece on and then brace it over to the pillar, or just do some angle iron sections from where they cut the rib over to the exterior skin. That'd add a couple extra inches of width on the interior but likely not be as strong. Still mulling those options over.
Also, exterior skin width is 73.5". I only measured about 3.25-3.5" depth in each flare. So total width before any insulation is going to be right about the 80" they advertise.
Main lesson learned so far: use a wire wheel instead of a flapper disc to take off paint on sheet metal. Sanding disc made some of that stuff real thin. Whoops.
'06 EB 350, Advanced 4x4 (soon to be MG), 6.7 Cummins + Allison, 24" bubble top. Bright yellow. The rumble bee.