Where does the neutral safety switch interrupt

Power in a 1971 Javelin?

You say power comes out solenoid post for the starter and it doesn't crank? Then solenoid shouldn't be the issue. Check the cable connections and battery voltage when you turn the key to the crank position.

Yeah, my next step is going to key to start position while testing power at the starter connection.

Agree with what everyone else has said. Start signal to NSS, from NSS to solenoid. If the solenoid is activating then the NSS is good. The other ways to test this are check the voltage at the S-post of the solenoid when the key is in start position, and second is to buy a cheap solenoid bypass switch. I have a couple I've collected over the years because they're just so dang handy to have. Always check the condition of the connections, there is a lot of current that needs to hit the starter and a bit of corrosion can cause a lot of headache. If you have voltage out of the solenoid on key-in-start but no action at the starter at all the most likely culprit would be the starter. I've seen quite a few solenoids that have bad contacts internally so you'll get 12v out but no current capabilities. One thing that I've seen a few times as well is starter grounding. Since the starter grounds through the engine ground, dirt and oil can cause resistance in that system and cause the starter to not activate and check the engine to chassis ground. So make sure the starter is tight and it is clean where it bolts to the transmission. As a result of these things, SOP in my shop has always been to remove, inspect, and clean or replace all connections in the starter circuit every time we worked on the system. It's just good practice.