Please be more specific.
The ballast resistors shouldn't have anything directly to do with the voltage at the battery.
But without seeing how your '75 is wired, its only a guessing game.
Which ballast was replaced? - there originally were two .
What does the gage in the dash actually show (5 amps? 40 amps?) and how long and under what conditions (idling? driving?)?
Before starting was the battery fully charged using a charger ?
Also measure the alternator output voltage is at the same time you're taking the reading at the battery.
