I just completed a shortbed conversion on my truck by cutting behind the cab. I incorporated a 2.5" step up in my frame when I shortened it, so I had to do it behind the cab due to clearance, but the process would be the same with or without the stepped frame. I have photos and details in my build thread (see signature). I based my conversion on the one done by 65standard
here where he does a straight cut on the frame just behind the rear body mount. If you are replacing the crossmember anyway, that is a significant portion of the work that you will be doing regardless of where you cut the frame. To me, cutting and rewelding the frame was actually pretty easy, it was relocating that crossmember that was a PITA.