I've been reading about headliners done for single cabs (great job) and decided I had to do something about my 75 SC. First here are the pics, then I'll give the details.
I pulled a headliner from a mid 80s SC that I figured was in good shape. Of course, there are the differences, like the visor mount and the dome light. Since I was only interested in getting a headliner in, I left the visor mount holes, but grabbed a dome light and installed it as non-functioning. Maybe later, I'll make it functional.
If you want to install this as a one piece item, it MUST be a two man job! I was by myself, so I opted to cut the headliner in two to make the install work. I also made a cover strip to take up the gap. This works for me.
When measuring for your SC, the most important item is to mark your trim, remove it and re-assemble it to make the correct pattern on the headliner. You will have approximately 4 inches front to back and a 1/2 inch on each side that will need to be trimmed. I used a sheetrock knife for the initial scoring and then scissors to finish the job. You don't need to have the material overlap, as it will tuck inside the trim.
This job took about 2.5 hours total to install. The biggest hassle was make sure it came out of the donor unscathed and then transport it home. Note: Save yourself some time and DO NOT try to slip this into your SC for an initial fit!
Like I said, this job worked for my beater and I can live with all the flaws that you see in the pics. Most will opt for the one piece install, and it should look great. Good luck with your attempt.