My educated guess would be the battery. Education is from The School of Bad Experiences. I rebuilt the top end on a '63 I had...new cam, lifters, pushrods, rebuilt heads etc. I only had it down for a few days and was scratching my head as to why it suddenly wouldn't turn over. I checked, double-checked and triple checked all connections. Jumping also wouldn't work. I pulled one of the batteries from my Excursion nad presto....started right up. After I finished adjusting valves, I drove it to the parts store and had them test and confirm the battery was bad.
