Assuming that the voltage varying is not a result of a looping idle, this is
my advice.Cars of this age start to have problems with electrical
connections. What you need to do is disconnect the battery. Where ever there
are wire terminals bolted to something like the starter, alternator and the
many grounding points is remove the bolt or nut and clean the terminal. I
like to solvent clean first then polish off the tarnish with 1500 or finer
sand paper. Clean the contact point on the stud also.
There are grounds under the carpet under the seats too. These are probably
ok unless the carpet gets wet.

You may want to start with the battery cables. Make sure that the contact
point to the battery is clean. I have seen them very rusted. If too bad just
replace them.

It may help to clean the connectors. To do this, open the connector and
spray with a penetrating oil like wd40 or lps. Then seat the connector and
remove several times. Blast the oil out with compressed air. Spray again and
repeat. There are many connectors so use judgment as to what you think may
be causing problems.

Would check the alternator. Rebuilds are notorious for failure, especially
in that series of cars. The fact that the alternator is tucked in the back
doesn't allow there to be proper cooling for it. A friend of mine would only
get about 6 months out of an alternator. I have only ever done mine once. I
took mine in to a reputable starter/alternator rebuilder. He used silver
solder to handle the heat better, and if there is a prob, he stands behind
his work, instead of giving me another mystery reman.