I have a 1960 GMC 1/2 ton with the 305A V6 in it. It has been in my family since new, and I feel confident it has not been changed significantly from stock. Whenever I search for spark plugs, I am given two sets of choices: plugs with 3/8" reach and plugs with 3/4" reach. When I inherited it, it had 3/8" plugs in it and I have continued their use because I am not sure of the clearance to the piston head. Does anyone know for sure that a 3/4" plug would be ok? The closest I have gotten to an answer is on the 6066GMC site which recommends a Delco AC4 (3/4" reach) for the 305E, which I believe was after 1964. Were there any clearance differences in the combustion chamber from 305A to the 305E? Thanks for any help or input.