I'm just now reading your post and here is what I can tell you. I have an 84 C10 Silverado for which I ordered a complete interior kit (81-87) from USA1, about a year and a half ago. My truck had the original fixed bench seat which had the seat belts passing through the seats. I needed a new seat cushion so I also ordered that from USA1. The only one they offer has the cut outs for the seat belts. The kit is pricey but it is jamb up and very, very nice.
I really wanted a tilt back seat so I went to my local square body parts dealer and swapped my seat, even steven, for a tilt back seat. It needed a new seat cushion but I had one on order. I'm not sure what year truck it came out of but the frame mounting holes were identical to mine and it bolted right in for the trip to the body shop.
Over a year later, the truck was nearing completion so I picked up the seat and took everything to the upholstery shop so they could install my new seat cushion, along with the saddle tan cloth and vinyl upholstery.
I got a call from the shop, two days later, with bad news. They told me that there was no way the upholstery was going to fit because my seat was about 4" longer and they could not stretch or lengthen the upholstery. They offered to make a new seat cushion, with my choice of density, and to duplicate the cloth/vinyl upholstery. I gave them the go ahead and it turned out great!
I don't know what the hell happened but I've spent a boat load of money and now I'm stuck with a seat cushion and cloth/vinyl original saddle tan upholstery kit ($430.00) plus shipping
. I suspect my seat came out of an 80 or earlier model.
Good luck with your project.