IIRC, it's 3/8", but don't bet on it.
BTW, I love the signature. They probably saved under $20 per car by not galvanizing the undercarriage. Of course, by the time cars were 5 years old in those days, high school kids were buying them for $300, because cars with over 75k miles were considered worn out junk. The warranty was 12/12 (3/3 on K codes). I once asked a Ford engineer, who helped build the car, and this was what he told me. When I asked him why, he told me to multiply $1 per car times 1,000,000 cars. Oops. Ford really did count things that close.
