An 8v battery was a common solution to starting problems on 6v vehicles (often used by farmers who had 8v tractor batteries laying around?).
As stated above, this "solution" is an admission of defeat and of failure to solve the real problem(s):
1. weak/old/ready-to-fail starter
2. weak/old battery - a 6v Optima battery solves this one
3. Wrong (too small) battery cables - 1/0 or 2/0 (0 or 00) gauge cables are preferred (2 gauge were original for 6v, and 4 gauge for 12v)
If you are sticking with 6v, get the right cables. I bet that 90% of people who have a 6v vehicle have wrong-gauge battery cables (especially the ground cable), unless they have already replaced the cables.
I am speaking from first-hand experience (I foolishly put off getting the correct cables for about 5 years - and had to do a lot of push-starting when the engine was warm).
