I can't imagine those bulbs are causing the problem, since those circuits aren't on when the car is started, leastwise if you're not leaving your lights on and your wiring isn't fouled up. I'd still want to change them out since they're liable to be half as bright running on 12V as they ought to be.

What brand battery is under the hood, and what voltage is it sitting at at rest? You should not be having starting problems with the small amp load you've added to the system, especially on a gasser.