C43 plugs were made for trucks running full throttle constantly. Back when these trucks were used to work with, they were under powered an constantly driven at full throttle. Hotter plugs would cause preignition. I seriously doubt that anyone works these old trucks the way we did 50 years ago. 44 is a good choice for highway as well as some city driving and 45 is probably better for most people's driving habits. AC 44 plugs appear on eBay quite regularly. C44 appear very rarely.