['68OrangeSunshine]

I did this back in the mid-'80s, before the internet, and this site, and Period-specific vendors like LMC.
I took the OEM dashpad off the cab and chipped the very brittle chips of old Fawn vinyl from the foam backing. I was careful not to gouge it. All the old vinyl came off with a putty knife -- eventually.
I had just had a pro seat shop do the upholstery on my seat in Saddle vinyl. I saved a strip of material ~7' x 1.5'. I sprayed 3M 77 contact cement on to the foam and then the back of the vinyl, and worked it up from the ''leading edge''around to the back, carefully stretching as I went. I had to tuck in sort-of "hospital corners" at the ends, on the undersurface. Excess was trimmed with a razor knife. I bolted it back in the dash and it's still there in my '68 C/10 Stepside.

I tried a blue one from Brothers, for my other truck ['71 GMC Jimmy] years ago ['90s] and it looked OK, until I tried to cinch up the last nut on the extreme [left] drivers side. It wouldn'd start. Had to pull it off, only to discover that that stud was an FD --all slag and no threading. How it got assembled, past inspectors [if any] IDK. I sent it back for a replacement, and they were good about it. But I had to install it twice. Only reason I didn't use my own method on the blue truck was the under-foam was compromised on the OEM dash pad.