Bad ground?

A 1975 should have electronic ignition, not points. It sounds like your problems are with turning over the engine with the starter. I hope you bought a mini-starter. If not, you should next time. You must have a multimeter to diagnose anything electrical (free at HF). Measure from the biggest stud on the starter to the case of the starter, as you crank the engine. If <8 V, find the excessive voltage drop(s). They could only be in: big red battery cable to starter, starter case to transmission, transmission to engine block, or engine block to BATT-. I don't know about 1975, but most Darts ran a thick "return" cable from BATT- to the engine block. Next time you visit a junkyard, maybe get both the mini-starter and BATT- cable from a Dodge 80-90's V-8 truck or Jeep GC V-8, like I did. That cable has multiple wires to ground the engine, frame, alternator case, ... There is a reason they did that.