Joe I have a K3500. It weighs more than 5,000 pounds with a GVW of just over 8,000 pounds.
My '52 6400 weighs about the same, but has a GVW of 16,000 pounds.
My P30 (forward control 1-ton chassis) has a GVW of 12,000 pounds. It's a motorhome and I'm sure weighs a lot though I don't know how much.
So if I load my 6400 like it was designed to be, and then try to stop it with brakes designed to stop half the weight (my K3500), I could be in real trouble. And the brakes from the P30 might be better, but not better enough.
If you convert to disk brakes, make sure you are using stuff designed to stop something with your true GVW. Otherwise you might be unpleasently surprised someday.
Going downhill in an big truck I've always made sure I was in a low enough gear the truck could not go faster than my brakes could handle.
Be safe! Over engineer your brake conversion.
If you take brakes from a 3500 adjust your GVW to match the 3500 donor.
