"68 Barracuda dash/radio lights

Before I can say for sure I would need to know if any other electrical changes with engine rpm.
Odds are the entire electrical system is suffering from low voltage, its common and correctable. Afterwards the dash lighting might still be less than bright as it should or could be even though the radio and shift indicator lights burn bright.
Keep in mind that of the 8 bulbs in the inst' panel, only 4 are illumination. Other 4 are indicator bulbs.