A self tapper will just vibrate and walk out. Buy a nut cert kit. Drill the correct size hole for the cert, you will install a threaded insert with the tool, and can install a bolt to do the repair CORRECTLY !!!!
The new foam is far superior than anything you have that is 25 years old. Secondly I replaced the seat bottom in my 1999 k2500 suburban, gray cloth, and it matched 99% exact and that is all you can hope for. Again the material and the construction is way better than the stuff 25 years ago...
Check the drivers side switch and wiring in the door jam first. If there is a broken wire or a bad switch, the passenger side will be dead also. Check for power at the actuator while you have the drivers door panel off.
In all this I do not see that you changed the fuel pressure regulator. This a notorious problem for the vortec 454. This will give you a black spark plug, poor idle, and hard start.
All eight of mine have custom hand painted pinstripes and it looks awesome. Compliments always. Remember, do it right once, and only bleed once !!! You can never have too many.
You will need to regear. 1996-1998 gmc sierra z71 or 1996-1998 silverado z71 g80 rear. It's a 14 bolt rear, with 6 lug axles, 11" drums. Sound like this is what you are looking for.
2019 GMC K3500 SIERRA DURAMAX QUAD CAB. GM quality is terrible. Rear window installed at factory with a gap in the butyl.... big surprise, interior full of water.
1996 one air bag, 1997, 2 air bags, large. 1998 small, 2 air bags and pass lock. Better to find a donor column. $150 is the going rate and a simple swap.