First off, stop thinking about the vacuum advance. I can disconnect mine and rip 4 gears of rubber. The point is, when you go WOT, vacuum drops and the vacuum advnace is supposed to go away.
Do you check again and rev the motor up beyond 2500. Go to 4000 and see if you have any more advance.
Take the cap off, remove the rotor. Now, turn the rotor arms, you'll see exactly how the mechanical advnce works. You should see pins engaging in the top base which limit the travel of the whole assembly. Are there bushings on those pins.? If so remove them. At the same time, you'll want to put a heavier set of springs in there. The springs should be readily apparent. You really want to spread the curve out a bit and have it ciontinue to advnace until approx 2800 with cruising gears or 300o-3200 with 3.73 or greater gears.
It might be good for you to find out who the distributor guru is in your area. Ask the racers. Take the HEI in and have them recurve ir properly.
Not-for-nothing, but I have found, more than once, that a stumble or hesitiation on acceleration is caused by faulty ignition wires.
But u need to resolve your lack of advance first.
EDIT: Dont do ANY carb tuning until you are getting a full 34-40 degrees of MECHANICAL ONLY advance. ( Your initial with mechanical curve in distributor).
[This message has been edited by 427TRI (edited 09-25-2000).]