Have you checked the total advance(vacuum advance plugged,3000-3500 rpm)?Total should be limited to 34-36 on pump gas,but there are always cases that contradict this.Quite often the setting that produces the most low end torque and throttle response is too much at wide open throttle,and it can detonate under full throttle,even though you don't hear any pinging.I don't know how much mechanical advance your distributor has,but most stock distributors need the cam rotated to the shorter position to run that much initial advance.Check it out with a timing light and let us know what you find..
I find this timing question interesting. It seems that the theory now is to throw the timing light away and advance the distributor until the engine pings ( the aduep theory) and then back it off a little. In regards to the last five engines I have rebuilt (with new dampners) following this theory poduced an initial timing of 21 to 25 degrees. Let's look at factory specs regarding the distributor curve on a 65 4V 289 distributor C5GF-12127-A. At 2000 dist. rpm (engine rpm 4000) mechanical advance is set between 8 and one half to 10 degrees advance. Vacuum advance at 1000 dist rpm (2000 engine rpm) is set between 8 and 11 degrees (20 inches of vacuum). Using the ADUEP theory,we are now looking at a max advance of 35 degrees mech. and 36 degrees vacuum advance. Back in the sixties when these cars were new, this was not advocated and to my knowledge was never done. I don't think you would have got away with it. What has changed since then. The only thing in my experience is the gasoline. I think it takes this new gas about 15 degrees to make up its mind if it is going to ignite or not. I haven"t thrown my timing light away and I still set at original specs. I don't have heating problems and I am not pecking away at my pistons eating holes in them. Besides that I am hard of hearing, and unless my wife is along, I couldn't tell if the engine is pinging or not. She can't tell at 4000 rpm's either. This is my two cents worth.
I am sorry if I missled you. I was talking about distributor curves. I will condense my point. If you have 18 degrees initial advance and you then set mechanical distributor advance to 10 degrees max and vacuum advance to 10 degrees max, you now have the possibility of 58 degrees advance. Distributor mechanical and vacuum advance values double at engine rpm. Example: At 2000 distributor rpm we set mechanical advance at 10 degrees. Engine rpm at this point would be 4000 rpm and the advance would be 20 degrees plus your initial advance.