Hard to start, intermintant spark

I just completed a frame-off resto on a 63 belair. Everything machanical and electrical is new.
The engine is a new 350 which ran great and then started to miss, then not start. I've changed the HEI dist. Starter, new plug wires, confirmed timing and unhooked all of the wiring to the alternator and the rest of the car. I still have great compression in all of the cylinders and everything is moving the way it should. I am dropping to just over 9 volts while cranking. How much voltage does the HEI need, and what else could be causing this problem. This is the 3rd HEI dist. All with the same problem. Fuel is OK.