AJ, Your thinking of using the green cab roof on the red body?
I'd like some 'close up portraits' of the rust area's in the red cab and around the windows, front and back, of both.
Have you gone around with an ice pick and tested for your weak spots on the red cab?
Looking at the back wall pic of the green cab I see lines just above the belt line and then radius up leaving the original bottom of the back window to use as an alignment point.
But Like Donf says make sure it's needed first.