Need Advice - Cut the Top or Not

[Feldsine]

I'm starting to think about fixing the rust along the top of the A pillars and the roof on my 71 C20. As the pictures show, there are gaping holes. The rest of the truck is very solid, only surface rust and no rot.

What I am thinking of doing is cutting the top off and replacing it with the top off my 69 C10 donor truck, which is in great shape. The plan I have at this point is to carefully measure and cut the A pillars about 6 inches up from the bottom, and cut along the seam on the back of the roof and around the B pillars to separate the top.

What I am not sure of is if cutting the seam is going to allow the top to separate. It looks like from the inside of the cab, that the inner panels are just butted against each other and should separate without cutting???

Would it be better to cut the top off lower say mid rear window, or below the rear window or some such place?

I'm thinking since I have a donor top in good shape that is a better way to go than trying to fix the hole in the roof and tops of the A pillars...???

I'm not body work guy, I have a friend who is going to help, but he also is wondering what is under that seam. Would appreciate the guidance of any who have gone down this road before.