A few things to check,
Have you looked at the exhaust to be sure there are no restrictions. This
can kill power.
Check the fuel pressure with a gauge
You say the vacuum advance brought it from 17* to 50* at idle, this is not
I would expect to see more vacuum at idle with that setup, a leak or a
plugged exhaust or late timing will cause low vacuum readings.
Check the voltage at the HEI, it should be battery voltage. That car
originally had a resistor wire feeding the coil and the running voltage would
only be 9-10 volts. This will reak havoc on an HEI.
Artificial Intelligence is no match for Natural Stupidity