Easy stuff first- Measure the voltage between coil and good ground with the key 'on' and then in the cranking position(a helper is handy and disconnect the big coil wire!)and see what you get. That will help narrow the problem down. If the voltage is low both ways, the issue lies on the feed side of the switch. If it gets 12v cranking and low voltage in run, the problem is in the 'run' circuit, and vice versa. The switch itself is a possibility either way but start by checking connections and finding the point where the voltage is dropping. Firewall relay, fusible link through that pesky bulkhead connector, through the ignition switch and back into the engine bay....getcha the diagram and a meter and take your time, you'll find it.
