I posted something similar a week or so ago but didnt get much help. My 1972 302 has HEI 50K Volt. I thought it made sense to get early 80's Mustang plugs as they had HEI. Thought 1977 MAV plugs would work too as they had electronic ignition then. Problem is the plugs for 80's Stangs and 1977 Mav are the thin ones and are too small to thread on my car. What plugs should I get? I am so close to putting the motor in and really need to fire it up.
I used Autolite 45's on my 302...gapped them at .040-45. They worked really well with the upgraded ignition!
I had an MSD setup complete with Dist in my 302....currently I have a 351w with the same MSD system...in addition to the two-step and the MSD spark advance/retard box. I upgraded to a set of AFR heads 5 or so years ago..and still use Autolites!
Sounds like your heads take the large 13/16 " old style plugs. The newer "peanut" style 5/8" plugs are too small to thread in, as you found out. You'll just have to go to the parts store and find plugs that fit the heads, then start playing with the heat range until you find what your motor likes best.
Get plugs for a 75-76 302. Really doesn't matter anyway, your old plugs, if still good, will do just fine. Just open the gap another .010.
