I just ran a post on that same carb. Are you running a Performer manifold with the carb?, or an LT1?
I went nuts with mine, replaced ignition, module, complete distributor. I readjusted the idle mixture over, and over. Each weather change, the van would run differently.
I am presuming you are not computer controlled, or you would not have that carb.
The HEI has an interesting quirk. The coil must be grounded or it fries the rotor. That is number one.
Second, as it sounds like my issue, check vaccum. You are probably timed to peak, so any drop in vaccum will make a big difference.
First, check air filter, obstructions in the air filter and intake portion of the carb. Second, check the power brake hose connection. It is in front between the idle adjust screws. Next, check the larger vaccum hose connection to the distributor.
If you have the performer manifold, you also have an EGR adapter, and carburator offset plate. Under the offset plate is a thin gasket. Above the offset plate is the 1" gasket. My carb had a gap caused by a thin washer under the right front stud. I am going to speculate, the 1" washer is the culprit.
My issue came on me suddenly also. I went nuts. I only found out my issue by trying to put a Q-Jet on to replace the Edelbrock.
I also had an idle screw back out on me. I had to take out the jet, put more tension on the spring and put it back.
To adjust you idle screws, turn them in to close, not hard, just closed. Open them out 1 1/2 turns. That should be your idle adjust point. If idle speed is too low, turn one out untill your engine goes to idle speed. Match with the other one. Using a tach, turn one untill the idle speed either goes up 50rpm, or down 50rpm. Match with the other one looking for the same gain or drop at your idle rpm. Same as a Q-Jet.
Also how is the torque on the manifold? Did you retorque the heads after 500mi?
Is your converter plugged? or plugging up?
Good luck, I know how frustrating that one is.