It should come off with a deep socket and a some good leverage via a solid ratchet etc. A really good idea here is to get it out fully and clean those connections with a wire brush etc, wire connectors , stud , nuts and all to be sure you've solid grounds and do the same to the starter solenoid connections too.Plus check the wires for internal corrosion bec that age of a vehicle especially if subject to extreme temperatures or extreme climates are highly likely to have those wires especially the main thicker wires starting to corrode etc.
There is some much better stuff out then wd40 if you check with the parts stores etc there is a great product out that is used by jet engine mechanics, I just can't remember the name lol If you do use such sprays let the spray totally soak it and allow it time to really settle in and do its job give it as many hrs as you can maybe 12 or 24 hrs and go back spray it again soaking it.
Hope that helps and best of luck.By the way it really would be a good idea at the same time to check and clean all the connections from the power junctions , fuse box , battery , alternator , engine block ground and all grounds.
