He told me to just unhook it because gas these days doesn't do well in these engines.
I always cringe when I hear things like this. The mechanic doesn't know what he's talking about and doesn't want to work on your car.
Detonation (pining) at WOT is consistent with low octane gas, timing too advanced, or both.
Set your timing to the service manual spec. (5° BTDC) at 550 RPM.
Use the table in the 1969 service manual to confirm centrifugal and vacuum advance operations are within spec.
Your car likely has factory AC which means you should have a thermal vacuum switch which switches the vacuum source to the distributor between ported and manifold depending on coolant temperature. Here's a description of some of the extra stuff AC cars come with:
This stuff mainly affects engine performance and combustion chamber environment at idle.
At idle, the air/fuel ratio is less than ideal because exhaust gasses are not fully expelled at low RPM making the mixture richer. Hence there are more unburned hydrocarbons during idle. Setting initial timing low causes the engine to operate at a higher temperature which is desired to help reduce unburned hydrocarbons as well as carbon deposits. If the temperature gets too high, the thermal switch changes dist. vac. adv. source from ported to manifold.
If your engine is still in its factory configuration (e.g., same intake, same carb, same ignition, all emission equipment present and working, etc.) do a tune up with the factory recommended heat-range spark plugs. Use an emery cloth to dull any sharp edges on the negative electrode which may contribute to pre-ignition detonation. Fill tank with high octane gas then do a week's worth of test driving.
Let us know your findings.