72 Chevy c20, starts, then dies when key goes to run position

[TNsurveyor]

Truck ran when I parked it in the barn back in May. Went to start it last week, fired right off, but when I let go of the key, it just dies. I can pump the throttle while the starter is engaged, and it will rev up, but immediately dies when key hits the run position.
I installed a new tumbler, no change in symptoms.
I installed a new ignition switch, no change in symptoms.
Have inspected the wires, cannot find a short anywhere.
My accessories have power when the key is in the run position, and when it is in the accessory position.

I need some help with this. Any ideas will be greatly appreciated.