Cut the bad one out till you get to good solid metal, then cut the good one to match.
I built a rotisserie for my cab that pivots around the centers of the doors, but you'll probably want to go the "normal" way, pivoting around the front to back centerline. I think for that repair, I'd consider standing the cab on one side.
Kevin First car '29 Ford Special Coupe #2 - '29 Ford pickup restored from the ground up. Newest Project - 51 Chevy 3100 work truck. Photos
I don't see a reason to spend money/time on a rotisserie for the 59 cab. If you are talking about the inside upper surround, you sit on something like a seat and work sitting. The only possible reason would be for something under cab. In which case, just sit it on it's back or nose. Everything else comes off truck. Rotisserie is a waste of effort for nothing. They are for unibodies/El Camino/cars or possibly serious cab floor/support repair.
Again, if you are doing it for the inside surround, a rotisserie can't give you a better angle than just sitting in the cab on a milk crate.
I hope that you will be able to drill out the spot welds across the top pinch weld and leave the outer pinch weld in place. Then cut where necessary with a .060 thick angle grinder wheel. There may be spot welds under the front of headliner trim. Then it would be just small cuts at the sides to butt weld. With all that rust, it not going to be easy to get a tight butt weld fit up. Removing spot welds will reduce the butt fits greatly. So normally you just cut out the rust, no more. But in this case there are other factors possibly. See what MNSmith says.
Keeping the windshield opening to spec is critical.
FYI: The wheel shown on the 55 is nub style hub cap retention. If you want to be original, keep any wheels you have that have hub cap clips. Nubs are worth keeping also but not correct for your years. Give'er hell aye?