will somebody please look at your alternator

'68 is a single-field system.
If you DO have 2 field terminals, it's technically for a '70-up application, but grounding the "spare" field teriminal will do no damage.

If you have doubts about it charging, with the engine running, CAREFULLY touch a screwdriver tip to the middle of the alternator pulley, if it's charging, the screwdriver will stick to the pulley.
Hold onto it tight, the magnetic force can sometimes surprise you!

Do you have points or electronic ignition?
I'll assume you have points and say that the wire from the distributor (which is USUALLY black) goes to the NEGATIVE side of the coil.

Mark.