You have two ignition circuits, start, which in a stock application, bypasses the ballast resistor and feeds the coil 12V during cranking. The other is run, which feeds the coil thru the ballast.

With your DIY HEI setup it appears you only have run connected so it's only getting voltage when letting go of the key. You need to verify both circuits feed the HEI so that you have power in start and in run