Well I was having the same crank no start condition you were describing on my 77 D150. Would crank but would not fire, no spark from coil, would try to fire just a little bit when the switch came back to run. So I checked around and could not find the issue and condemned the switch at the culprit. Put a new Standard switch in. Same issue So I kept digging around and found I had a poor connection at the bulk head connector at pin 18 not letting enough voltage pass through to let the ignition fire while cranking the truck. Made sure I had a good connection and BOOM the truck fired right up. This problem had every once in while would do this but then it would start so I never chased it to hard. Drove the truck to Carlisle this year with no issues on the ignition. Got back parked in the garage and then it never fired again until I chased all this down and fixed it. Might not be your problem but that is what it was on mine.