Disc brakes on the rear in my experience always came with disc on the front too. Never seen disc on the rear and drums on the front (factory) and not sure it's a good idea to do so.
Are you also considering disc brakes up front?

Depending on the engine you swap it will/can have a different comfortable operating RPM.

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-