The 3/8" plug may have been the factory issue. If a longer reach plug doesn't interfere with the piston, go ahead and use it. It places the spark closer to the center of the chamber for a better, more complete fuel air burn. It may even increase power and fuel economy.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)