There are more threads on it, but here’s one about that master cylinder swap:
88-94 brake lines are how I deleted mine and I've never regretted it. That's the exact route I'm fixing to go. Run the old style lines, a proportioning and call it a day.
www.gmt400.com
The Cliff’s Notes version is that the GMT800 master cylinder isn’t really appropriate for the rest of a stock JB5 system in the GMT400 without upgrading other things. Been there, done that. The truck had better pedal feel with a 800 MC but felt like the pads were made of smooth glass. Greatly reduced stopping power. I ditched it and put the stock style back on.
The pedal feel is not impressive compared to anything modern, but mine work best with the stock master cylinder, an ABS bleed followed by a pressure bleed, and properly adjusted drums.