If he originally had a single circuit MC then the question is what new valve or distribution block if any should he now use with a dual circuit MC?

I say find a proportioning valve from another GM truck with drum/drum brakes and use it with your new dual circuit MC.
What did the 68 GMC use (your MC donor)?

But I don't have any technical reasons why or what you should do.

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
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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-