I have been installing Ford style pre-start solenoids on all of my early Nissans, this has been taking care of a lot of intermittent starting issues. Take the push on terminal and see if you have 12 volts when the key is in the start position and and lever is in neutral, then take that terminal off and install a ring style terminal on the wire, then hook this wire to to the small terminal on the solenoid, put another ring stle terminal on the other small terminal of the solenoid and run that to the chassis for ground, then take a 12 guage wire from battery positive to large terminal of solenoid, then run another 12 guage wire from the other large terminal of solenoid to the starter energizing terminal on the starter solenoid, where the push on terminal originally went. The heavier guage wire has cured many starter issues for me lately. I have installed a couple of new starters that just went click when you hit the key and the heavier wire has cured the problem. In fact Cat started doing this as an update for their GM 4.3 starters that were being replaced unnecessarily, they added a heavier guage wire from the starter relay to the starter.