From 45+ years of work in the electrical/electronics (now retired): The larger the wire size, the more "oompf" available to the starter; so, 00 (2/0) is better than 0 (1/0) is better than 1 is better than 2 is better than 3 is better than 4, etc.  And shortest is best of any given gauge size wire.  Go with shortest and biggest wire available/affordable that will "form/fit/function" for the application.

Regards,   JLB

55 Ford Crown Victoria Steel Top

If there's a Tractor Supply near you they can probably make up what you need for a good price. Many online sellers as well. Many people suggest 00 or "double ought" for 6 volt systems.

Check out Rock Auto. Cables are listed for 55 and they are not the same as listed for 57 (12 Volt) so I would assume they are heavier Gauge for the 6 Volt System. 

  

Easiest way is like Lou showed, reproduction parts. The other way is to go to a welding shop and buy the appropriate size welding cable. Then solder or crimp the right ends on. If you don’t have the necessary tools, I’d go the repro route.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

http://www.classictbird.com/Charging-Starter/products/44/3/0

I read another article about cables.  For 6 volt starters you need 0 or 00 size cables.  Is that correct?   I have suspected my new batt. cables (size 4) and my jumper cables as being the culprits.  Cant buy 00 or 0 at auto parts stores.  Your thoughts please.