Mark,
You may not have noticed this in my post above: "The pickups and panels are the same from the doors forward, so the same things apply."
So you don't need a panel truck as a donor. Any pickup cab from 51 or later would work. You could theoretically slice that donor pickup cab and yours in two right in front of the seats and splice the two good parts together. I don't think I'd recommend that option though. It would be difficult to get everything aligned correctly and have a decent seam across the roof.
