I like under the floor, looks better and looks original, and I think less work.

Good plan on brake line sizes is to use what the vehicle you pulled the brakes from used. In your case a P30 or 3500HD chassis. I believe they were 1/4" from MC to proportioning valve. From there two different 3/16" lines to each front. To the rear a 1/4" and I forget if 3/16" or 1/4" on the axle to each side?
All this you can find by looking at one in the parking lot, or perhaps a friend has one complete you can scope out.

•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-