I agree, the voltage is a bit higher than what I would want to see. I like 13.2 - 13.8, but anything under 14.2 is fine bcefore I begin to get excited, although I have seen some specs say up to 14.5 is okay. I rarely see much over 14.2 on the First Generation Mustangs. The voltage you re seeing is not horrendously high, but there is no sense in risking perpetually overcharging the battery or sending more voltage than needed through the lamp filaments. A shop teacher of mine once said it a bulb needs to be replaced always check the voltage from the charging system in case the filament failed due to excess voltage. Although I never ran into a problem with charging voltage when I replaced bulbs, I felt it was a good habit to get into.
Before replacing anything it may be a good idea to check the voltage with one or two other voltmeters, just to make certain yours is reading about the same as the others. I rarely see the digital voltmeters give problems, but the older analog meters could cause grief after a few years of use.