Ok, first of all I read that you checked the timing. This either has the DuraSpark II Ignition System or the TFI-IV Ignition System. I am assuming it's the DuraSpark II Ignition System because on the TFI-IV Ignition System no ignition timing adjustment is possible and none should be attempted.On the DuraSpark II Ignition System the distributor has a means to advance the timing of the spark as the engine speed increases. This is accomplished by centrifugal weights within the distributor and a vacuum diaphragm mounted on the side of the distributor.
So with that in mind hook up a timing light and check the operation of both advances.
ok what happens is if the timing mark moves or stays where it is at after I do what you say???....I checked for exhaust restriction with my OTC exhaust backpressure gauge.