I have posted about this many times. Get yourself some 2/0 cable for the positive and negative battery terminals. Be sure the grouding side ( in your case the positive ) is bolted to something heavy as close to the starter as possible. Add a heavy ground strap from the frame to the block.. Also be sure all the connections are clean..Do all of the above and your 6 volt starting troubles are history..
An 8 volt battery will mask the inadequacies of your electrical system for a short time, you will have to address them eventually. Big waste of cash in my view..
I run a 6 volt Optima battery in my truck with 2/0 cable, the starter never fails to spin like crazy.
Pay attention to Kook's and MT's comments here.
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=3;t=000676#00 0003
