I don't know if this is what you wanted or not, but...
I just saw a new item in a catelog that was a complete replacement for the skin on your seat. It involves removing your seat vinal and replacing it with a new one that fits exactly like to original one did.
I also found a web site for a place here in CA that sell them as well. I used Google.com to do the search and then called them and it was an exact replacement fit custom made for the exact year of the truck.
The prices were about $150 for a bench seat setup and they looked great in the pics.
I know this is an older thread, but I have been looking for an upholstery kit for a bench seat in a 1975 F-250 Custom, and although some potential suppliers were alluded to earlier in this thread, none were specifically identified. I have also been unable to directly locate a supplier - everyone I have seen sells hard parts, seat belts, Mustang upholstery, and just about anything but an upholstery kit for my truck.
Soooo, if you know of a supplier for upholstery kits for 1973-79 trucks, especially the vinyl custom cab style seats, would you be kind enough to post it here. Thanks.