What distributor are you running ?
Is it an original unmolested unit from the 70s or 80s ?
Leaded fuel burned at a much slower rate than todays unleaded fuels.
So, the old dizzys advance curve (rate of advance) and amount of advance are too great for unleaded.
An engine will not develop full power as a result (i.e. quite a lot of power is "lost") when running an original distributor.
This is aside from the "pinking" issue.
I have the dizzys "regraphed" to suit the cam and fuel. Invariably, the vacuum advance is disconnected as it is just not needed.
I use Performance Ignition Services here in Melbourne... not sure where you are or if there are other specialist guys in other cities.
With Performance Ignition Services, I advise them what cam and fuel I will be using and they set the dizzy up accordingly.
Otherwise, any of the new electronic distributors made for unleaded would suit.
This may not be your issue, but it is generally overlooked.
Have seen 10% to 15% increases on the engine dyno just by swapping out to a regraphed dizzy.
That was with 351 and 250 motors, but the issue would be the same for 1600xflows or the 2l OHC.