I'd probably run 3/16 to each front caliper and over the rear axle.
1/4" for between MC and proprotioning valve and to the rear axle.

Use an original modern truck with similar/same components as your guide, what did they use? May as well benefit from GM's engineering.

•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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