I'm not familiar with the 57, but on my 48 you can simply use the clutch pedal with a push rod length to fit.

The 90 degree bracket from the step van is a good start to a bracket to bolt the booster inside the frame ready for that push rod.

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-