I have upgraded brakes on two trucks with the master under the floor. A 52 3800, and a 61 ford 350. Both trucks now have disc brakes at all 4 wheels, and both trucks have a pair of 2 lb residual pressure valves.
For the brake line to go to the right front, I ran mine along and on the back side of the transmission crossmember, as the original was.

Both front lines come from their own places on the proportioning valve, so no T-fittings needed.
The proportioning valve has only one line going to the rear, then it branches off just after the flex line to the housing.

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-