My truck is a 1964, your truck I believed is the same. The only thing I had to modify on the front spindles was the ball joints taper is different. I use the proper reamer on them them to match the ball joints (do a trial fit to determine if it is require to be modified). One other thing I had to do to the spindles was to shorten the turn stops, wouldn't turn enough and now it match the specs in the manual. One other note, when converting to disc brake you will need at least 16" rims for clearance, and it does not matter if you install a 5, 6, or 8 lugs disc.
I don't remember why I used the 1500 front calipers instead of the 2500. It fit perfectly. It might have been because I had a old one laying around and it fit, so I did installed new calipers.
The rear disc brake uses the 76 - 78 Eldorado rear calipers with parking brakes. This was part of a kit from:
https://www.tsmmfg.net/general-motor...rake-kits.html or
https://www.rusticdiscbrakes.com/pro...-8-lug-dana-60 for a Dana 60 full floating axle.
I want to keep the 8 lugs wheel, which is why I didn't convert my truck to a 5 or 6 lugs hubs. I also got the rims from the 82 GMC and the hubcaps (see attachment).