And I was thinking most were 3/8", thus the -6 part number earlier. Might be a little truck/car difference and I'm more familiar with full size stuff.
So long as it fits well you have the right plug..

•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-