I have a 48 with a 48 cab, and I'm switching to the later rear mounts. You can buy the missing reinforcement plates/brackets that go on the bottom of the cab from the usual vendors, reproduction. Mounts you need to find used and buy the bushing kit.
Drill 4 holes in the frame side rails and remove your old center mount.

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-