HEI conversion problem: no spark while cranking

All aftermarket and wiring changes aside... When you get a single spark as ign' switch returns from start to run, the fault can be the ign' switch. I would disconnect and check it at its harness connector under the dash. If good there, my next stop is bulkhead connector.
What I'm saying is regardless what we do downstream, we're still relying on a lot of the cars OEM wiring and components.