The commercial plugs were kind of like the Champion "Truck Plugs" They had a larger ground terminal.
I don't think you can even get the R44 plug in anything other than a tapered seat anymore.
An R43, R43S, R45, or R45s, will more than do the trick for you.
These old trucks ran at a lower compression, plus most modern gas is probably in the 89 octane range due to ethanol and EVP requirements. So, even going with a hotter plug won't hurt a thing.
