I recently installed a pair of retracting harnesses in my panel. The only way I found to mount them is to drill a hole in the inner wall (where you want the mount points) then cut an "L" in the sheetmetal about 3" from that hole, and peel the sheetmetal back. That allowed me to put a reinforcing plate inside the wall, bolt the seatbelt assembly through that plate, and then tap the "L" back flush with the wall and patch it with fiberglass (I'll be upholstering over the patch later). I did this for all four mounting points in the cab (top and bottom each side).
The kit I ordered (not ChevyDuty) came with a latch on a rigid plastic extension, which worked out very well with my bucket seats (from a '96 burb). I drilled a hole in the back of the seat pedestal (which took forever, very hard thick steel) and offset the latch assembly with a right-angle steel bracket. This offset was necessary to set the latch in the inside corner of each seat and maintain correct tension points. You want these to be a safety item rather than a potential hazard, so pay attention to where the belts cut across your body, and adjust the mounting points accordingly.
I should point out that the only reason I chose a retracting harness is because I have a panel, and need the belts to tuck up next to the wall when I'm crawling into the back. Lap belts can be bolted directly to the seat mounts and won't require any cab cutting; retractors must be bolted to the wall vertically to work correctly.
Hope this helps. Good luck.
