[jonzcustomshop]

you are pretty mich onto it.
if you cut out your sbw inner panel, and weld in the new bbw inner panel, it will fit right up to the sheetmetal that surrounds the old sbw.
I would drill a hole about every inch to inch and a half on your inner bbw pinch weld. then plug/spot weld it to the sbw sheetmetal on your cab.
when all this is done, just cut out the window opening from the inside using your new bbw edge as a guide.
grind all the spots smooth so that the rubber will fit over it.