Have you taken a compression test? Something ain't right. Can you post what the plugs look like?

I like the Autolite due to the shorter strap and less chance for detonation and better spark exposure. I run the 3924's in my 524 with 10:1 compression with no issues on 93 octane. I have a big cam carbureted engine and I idle for hours down a river and I have no fouling issues. But I do have a multi-spark ignition. I also bought an otoscope from walmart so I can check the banding on the strap to make sure I had the right timing and heat range.

if its an est with the square coil they are very hot but need a lot of power you probably need to add a relay i run a 540 on the rich side and never foul a plug but if it is weak you will

I run 3923s in most aluminum heads and never had a problem with fouling if tuned properly

round coil

I’m sure that old 502 was getting tired. I still feel like the carb leaks down when setting. The level in float bowl definitely goes down but I know some will evaporate.

with similar compression and pump gas?

Ive ran them with10-1 on 93 and at 8.4-1 on pump gas and at 13 psi boost with 93, one of my favorite "cheap" plugs, I actually LIKE the projected tip because they dont foul easy. I ran a set of 3923s in my 548 w prochgarger for 100 hours, pulled them out at 40 or 50 hours and they looked good enough to just put back in. LOTS of idling too, Smitty

Ran 3922s in my 11.4 to-1 street car on 93 (455 pontiac), seemed sputtery, went back to 3923's