Classic/Antique Car Repair: 38 Packard Starting problem, age of the dinosaurs, volt batteries

Question
I have a rebuilt 38 Packard Six cylinder. The engine will not turn over with the 6 volt battery. It will start and run fine on a 12 volt battery. The 6 volt battery is brand new (optima) fully charged. I would like to hook up all of the lights and accessories but hesitate to do so untill I can get this car to start consistently. Is there some type of switch that I can install and use a 12 volt for starting and then run on the 6 volt at least untill the engine loosens up enough to start on the 6 volt system. I am trying to keep the car as original as possible and don't want to convert to 12 volt completely.

Answer
Way back in the age of the dinosaurs when we used to build our own tow trucks, we rigged two six volt batteries in series (a 12 volt battery) mounted in a box. Then mounted a plain old Ford style solenoid on the side of the box with a long remote cord to operate the solenoid. Then ran the cables to the 6 volt battery in the cold non starting car. Turned on the ignition, engaged the cars starter and then hit the remote switch and presto, a 12 volt boost. Once the car started release the remote switch the 12 volt battery was disconnected. I'll bet that this is what you are trying to build. I hope that engine loosens up soon.
Brad