on seems like spark. I have had a wire break on the distributor that connects near the centershaft. I did not catch it till the 3rd time pulling the distributor trying all sorts on things on my old one.
Only other thing I can think of is their is a second oil pressure sensor/switch on some models that if truck has low oil pressure (it was next to oil filter underneath truck), it doesn't let it start. My 96 had a sensor that was failing that drove me nuts. Not sure if your year would have one or not.