Great minds think alike!
I just acquired a 'glass cap for the F150, somewhat damaged and free off Craig's List, with the intention of doing a fastback chop. I was planning on doing the necessary work to patch it all together in fiberglass.
Earlier I'd gotten an 8' cap that would have made for an easier chop (no or little material addition; just subtraction) but I didn't want the extra height and it's in nice shape so that one went to work on a long box truck.