Depends on where you are "losing it." I always try to eliminate parts and pieces and get the problem simplified. In this case the easy way is to test it by jumpering the starter solenoid, the two biggest bare terminals. That will jumper battery power direct to the starter. If it cranks OK, the problem is elsewhere in the wiring. If it doesn't you have now got the problem isolated to the battery and their connection points, the starter, and the minimal wiring from the battery to the big stud on the relay, and the solenoid wire going from the relay back to the starter.
Battery clamps can seem clean and not be. Get a good strong terminal cleaner. Cables can corrode internally. If you have a gun type thermometer, look for hot spots. Cables can corrode internally and become effectively smaller. Give a wrench twist to the block ground bolt, and to the starter stud connections.
Learn to use your multimeter to test voltage drop.
If this turns out NOT be be the problem, then you add the starter relay, the wire to the neutral start switch, and the wiring through the bulkhead via the ammeter circuit to the key switch, and the yellow start wire back out through the bulkhead to the starter relay AND THE relay itself.
The way you describe with the lights it sure could be bulkhead, OR JUST simply still battery clamps The number one problem with electrics on old Mopars is often the AMMETER circuit through the bulkhead connector. The connector separates into three separate connectors. Examine the terminals for corrosion, oxidation, and heat damage. You can get those terminals and redo them
IF YOU DO NOT have a factory service manual, run over to MyMopar.com and download one for free
