[Big Dave]

Looks like you about where I am.

My take:
Hinges on the cab is not the right answer. The hinges will inevitably foul the air flow and the cab structure is not solid enough for anything heavier than paper. I intend to use a stout front bulkhead to operate the hinges.

We have discussed how far to taper down. For my 80" long bed my new fastback will truncate at 8-10 inches above the tailgate top. That gives me a 12 degree slope where the flow should stay attached all the way to the end. Mine is now 17 degrees and I may be getting flow separation.