Epilog:

For possible future reference and help in others trouble shooting adventures....

All the new IMPCO parts are installed.

Note:
I called the IMPCO tech dept and they said the "Vapor Fuel Hose" that connects between the regulator and mixer should be at least 5/8 ID and preferably 3/4 ID.
Anything smaller could cause the regulator to not work correctly.

The same problem still existed!
An "apparent" lean condition off idle with eventual stalling all together after a period of time, usually 15-20 minutes.

The solution:

It was the IGNITION COIL

Old hot rodders saying:
95% of Carburetor problems are electrical.

After a run of about 20 minutes, it got rough, stumbled and died as usual.
I started to think what else it could -possibly- be.
So I hooked #1 spark plug wire to an external plug.
And I hooked up a timing light to the coil wire.
Started engine at idle and watched.
After 2-3 minutes at idle, the engine began to run rough........I could actually see the spark on the plug hooked to #1 wire get thinner and weaker, and more orange.
30 seconds later, the timing light stopped functioning!
5 seconds after that the plug stopped firing and engine quit.

Crank engine......no spark.

Wait 5 minutes, crank engine, starts back up.

Theory and conclusion:
My guess is the spark was so weak it was just enough to allow the engine to start and run, but with increased throttle, and added fuel, the spark was simply extinguished.
Spark was stronger for first 15-20 minutes of operation.

Coil did not get hot or even warm to touch.

Installed old, use, poor looking coil to test and engine runs
perfectly. Idles smooth as a watch with excellent pick-up and power.

Thanks for all the info and responses.

dk