How I installed a 90's, 14-bolt, semi-float axle in my 1971 C20 pickup. This swap maintains the original axle width (65 1/4") and adds 13" disc brakes. All parts are GM so it will be reliable and work great.
Axle = 1989 - 2000 Chevy & GMC light duty 3/4 ton with drum brakes. 2WD and 4WD are same width if 13" drum brakes.
Disc Brakes = Early 2000's Chevy & GMC light duty 3/4 ton. Calipers mounted on back side so parking brake cable routes correctly.
Other necessary parts:
1. Axle perches cut for 3 3/8" axle tube
2. Laser cut brake mounting flanges (1/2" thick)
3. Laser cut backing plate concentric rings (3/16" thick)
4. 3/16" custom brake line kit and 3/16 to 1/4 adapters at distribution block
5. 2X LH brake hose from 2001 C2500
6. 2X LH parking brake cable from 2001 C2500
7. 9/16" wheel studs for 80's 9.5" axle
https://www.youtube.com/watch?v=e-AF0CLiAQA&t=638s