I am having the same problem with a 66 390 that was originally a 2 bbl that has been changed to a 4 bbl,I have been trying to ween it off premium but even being a 2 bbl motor the compression is close to 10 to 1 so it looks like premium will have to be used. I added electronic ignition and have been playing with the jets in the carb and I think it will run on 89 octane decently but I noticed it pinging a little too much with 87 when its warm outside,it did not ping on the return trip when the temp cooled off some so I am thinking about making a dual snorkel aircleaner with hoses going to the core support to get cooler air in the motor. Jeff