If you keep the 4.56 and still want to keep up with traffic you need an overdrive transmission, that's why we recommended the NV4500

For disc brakes there is no kit available, but there are ways of fabricating something to work. I'm figuring and calculating on that right now for an AD 1 ton truck.
What is the wheel mounting surface to wheel mounting surface measurement on your truck any how, we might have a nice solution already (but it's not quite narrow enough for an AD truck)

Check this thread, although it wanders around there are some good thoughts and ideas in there. That's what I'm working on now.
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=603612#Post603612

Then if you want a newer rear axle check this thread
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=599879#Post599879

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-