Hey man I don't want to mess with your plan here or tell you how to ride your horse, but you shouldn't accept that cab corner the way it is.
The entire thing needs to be cut out and replaced because the P.O. didn't butt weld in the patch panel.
I know it is extra work but cab corners are cheap and the results will be worth it.
Tabco makes the best fitting patch panels on the market.
http://tabcobodyparts.com/