Ethan,
Another wire I would check is the large hot lead from the starter solenoid on the inside of the dash tower to the starter itself.
I had a 1650 I was working on was having starting issues. Someone before me (and not the owner) had fiddled with the wiring trying to fix the starting issue. They cut out one of the wires from the key to the voltage regulator, and had added an extra ground wire from the dash tower to frame, and then to the engine. When I replace the rotted ignition switch, and put a known good battery from one of my tractors in it, and the charger hooked up, it would either click, or crank over like the battery was almost dead. During this process, I happened to be looking at the back of the engine when I was trying to crank it over (had the fuel tank removed so I could see under the dash) to make sure there wasn't anything interfering or something mechanical I could see. One time when it started to roll over very slowly, I noticed an arc and a bit of smoke come up from firewall area. Where the wire passes through the hole in the firewall, it had been pulled tight against the hole by the extra ground wire. So most of the starting current was going into the firewall and to ground.... I replaced the heavy lead with one of the pre-made auto solenoid to starter leads from the auto parts/farm store. Once all the wiring was 100%, the battery charged and that new lead from the dash mounted solenoid to the starter, it spun over just like my tractors would. However, had I not seen the spark and smoke, it would have been difficult to find. I think the reason I saw it was, I was in the garage with the door down, and I happened to be casting a shadow where I was looking because of where I was standing. If it had been brightly lit, I would not have seen the spark......