Thats good news. Did you try cranking it over with the heads and valve train removed?
I guess depending on what climate the engine sat in the starter commutator could get oxidized/corroded or what ever copper does. That would give a bad contact with the brushes...
Have you tried hooking your jumper directly to the starter terminal ?
1960 F100 Panel 223 3 spd overdrive
1966 F250 390 4 spd
1957 Ranchero 292 3 spd T85 overdrive