Hi Arnold,
I cut all the straight part on the back of the cab. I cut about 2/3ds measuring from the bottom of the lip. So I cut maybe 1 1/4" leaving the top part of the lip. My figuring is there is enough curve left so I could mig weld the new piece in place without much distortion. Bad news is that I cannot planish because no access to that part without also cutting the top part of the lower cab panel. We'll see how it goes.

The cab is composed of the lower part and upper part. They join at the beltline. If you look behind the seat, you can see the curved part where they meet. Both halves are spot welded at the lower part of where they meet. If you try to put something up into that gap at the bottom you can probably push it in about 1/4". That is where the spot welds are. No 3rd piece. Spot welds vary from 3" to about 3 1/2" from each other.

Dennis

40 Chevy 1/2 ton