I'm not sure about the 455...but on a 400...here's what to do. I assume it would be similar to the 455.
Measure the voltage at the battery terminal and tach terminal on the distributor (while the engine is cranking). If the measurement is higher than 7 volts, you're OK to that point. If you're lower than 7 volts, you have a bad lead in to the distributor, bad bulkhead connection, or bad ignition switch. And, I'd check the ignition switch first because that's a moving part.
Assuming you have enough current at the distributor:
Do you have electronic ignition or points? If you have electronic ignition, check the coil with an ohm meter per the service manual (it would be impossible to explain it here). If that's OK, try changing the module. Also, check the cap and the spring in the center as well at the rotor. Sometimes the screws for the rotors work themselves loose.
If you have points....hey that was a long time ago for me! But check the dwell and the condenser. Also, check the coil...but I've forgotten how to do that on the old style ignition.
Or, you could just do a tune up and change the coil, module and plugs. That may or may not solve your problem. If it doesn't, you need to measure voltages......either isolate the problem or do like most mechanics do....change parts until it works.
The any gear thing is a big problem. Someone must have bypassed the neutral start switch. That switch is located on the base of the steering column near the toe pan. Or, the switch might be bad, but usually if they're bad the car won't start.....it's a purple wire and gray wire...thick wires if I remember the colors correctly (and that's from a 1978). This switch also works your back up lights.....do your back up lights work? It's possible that they do because one connector fits into two blades of the neutral switch to run the neutral/park start function, and another connector fits into two blades on the switch to run the back up lights. So, it could be that the neutral start function was jumped and the back up light function is intact. Interesting problem. If you lived near me I'd love to solve it.