I know what you mean about the interlock, my air valve is pretty stiff to push, but only moves a fraction of an inch, which suits me fine.
To prevent accidental starts inline with the foot valve I'll use a cable actuated air valve operated by the choke cable, so one must choke my Detroit Diesel before it'll start smile

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-