Were the lights always dim or did you change something (like the wiring) and they got dim? Have you compared them with other cars that are 6 volts? Try temporarily running a 12 gauge wire directly from the battery to the contacts on the headlights and see if they get brighter. Include both negative and ground circuits. This would identify undersize wiring, bad grounds and/or a high resistance light switch. Who made the wiring and did they know it was for a 6 volt (not 12) car? I am sure there are brighter bulbs out there, call around. Good luck. If all else fails consider a headlight relay or changing to 12 volts.
Charlie Stephens