I have had similar idea and instead of all this talk of relays and wimpy switches...
Use a floor starter switch like used for old tractors, it’s already sturdy, mounts to floor, and way more capacity than necessary to trigger starter relay.
Only need to machine a cap for it with 3/8-24 stud on top for original button.

I’ll find the switch part number soon and post it.

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-