74 Duster won't start

Since the starter relay never actuated with the NSS connected, there was definitely a problem in the "neutral sensed" circuit. That doesn't mean the NSS itself is bad (usually isn't). Once you bypass the NSS, the relay does actuate intermittently. That suggests another problem. Of course, your jumper could be erratic. Are you grounding it to a rusty screw, for example? If not, the problem might be in the upstream side of coil actuation - key switch, wires, seat-belt interlock, wires, bulkhead connector. The seat-belt interlock was 1974 only I recall. Why? Because it was so erratic that customers screamed about being stranded the first year with a new car. Hopefully a prior owner already bypassed yours (many posts here).