Hey David,
From what you said:" When you crank it over there is nothing until you let off the key then there is a little spark." It makes sence that the problem is the triggering mechanism in the distributer. When you "let off the key.' it allows the magnetic field in the coil to colapse and produce the higher voltage that is terminated at the sparkplug.
First check the wire from your distributer to the coil negative(-) side, for continuity. If that wire is bad, the distributer cannot "tell the coil to fire."
If the wire is good, as Bully had said, a component of the Duraspark (stator, module) is bad. My thoughts on that is the components are so old it would not be prudent to change one component and not the other. AND again as Bully said, might be a good idea to change to an aftermarket ignition system.
One final thought....have you removed the Dist. cap and checked and actually seen the rotor turn? I had an occurance where the dist. gear sheared the pin, and the disy was not spinning.
Good Luck , and let us know what you find.