Joe, Do a search here on the page. The 6 volt cable problem has been written about a few times.
Click here for one thread You need at least a size 2/0 battery cable on both the positive and negative side of the battery. I had mine made up at a shop that services larger trucks. Be sure to connect the ground to a spot as close to the starter as possible.
Also make sure you have a good ground strap or cable between the frame and the block.
I am betting that a cable change will solve your problem. My 6 volt system spins as quickly as any 12 volt set-up..
