You should consider diagnosing the actual problem, instead of throwing parts at it.

The good news is that your intermittent problem will manifest itself often enough for you to troubleshoot (it's the once-in-a-blue-moon intermittent problems that are a bear).

Arm yourself with a volt/ohm meter and a wiring diagram. A good online source for wiring diagrams is MyMopar.com.

Given that arcing the starter relay with a screwdriver works, you already know that the starter, battery, and large wires connecting them are not the culprit.

Your manual transmission 1970 Plymouth Satellite needs two other things for the starting system to function properly:

1. A ground through the clutch/neutral safety switch [probably dark green].

2. Power from the ignition switch when in the start position [probably yellow].

Check for continuity to ground (ohms setting on the V-O meter) at the appropriate connection at the starter relay when you stomp on the clutch. Lacking the ground, chase that circuit (firewall connector, switch, switch's ground).

Check for power (dc volts setting on V-O meter) at the appropriate connector on the starter relay when the ignition is in start position. Lacking power, chase that circuit (firewall connector, ignition switch connector, ignition switch itself).

Good luck. Electrical is not voodoo, contrary to what some might believe.