I have a sbc 350 and a few weeks back the engine stopped cranking. I went through and the ground wasn't connected connected to anything so I fixed that. Still the same result, I wired a push button to eliminate any problems with the ignition and that worked until a week later same issue. I replaced the starter, alternator and its got a new battery. Now I can just hear the starter and flywheel I think, turning very slowly.

 

Had a similar problem with mine and it wound up being a multiple part fix. Went with a ford starter solenoid and replaced all the wiring to and from the starter. Half of it was rubbish and the trigger from the key had a crystalised patch of wiring that would sometimes allow voltage through but sometimes not.