I've always heard it's not wise to have disc in the rear without first or also having front disc.
Can you think of any factory vehicle with front drums and rear disc? Not to say what the manufacturers do is the only way, but what they do is a good guide and pretty much will be safe.

•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
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-