no spark in start

Current takes the path of least resistance. In start that would be the large battery cable to starter and the starter. Everything else about the car runs on different much smaller wire. Then there's a fusible link, amp gauge, ign' switch, bulkhead terminals, etc.., to create more resistance in this path. What you have found thus far, you have less than battery voltage through all of that stuff, and lesser still when starter is drawing the lions share. There always was a back feed through the ballast resistor during start but that resistor reduced the backfeed/draw. A bypassed ballast resistor doesn't. I would need to study wiring diagrams for your model to know what circuits are getting full 12 volts during start that should not.
Relays are used to get improved voltage to headlights. Same approach would get improved voltage to charging and ignition systems.