I will post only a brief opinion regarding going to eight volts to "solve" a starting problems.
Farmers often used to do this in "the good old days", because they usually had an 8v tractor battery in the barn.
In general, this method is avoiding figuring out what was wrong with your 6v charging/starting system.
Did you install the 8v battery to work on a positive ground GMC electrical system?
What voltage regulator did you use?
Did you clean-until bright all sides of the various ground cable/straps connections?
Maybe someone knows how you should be wired from the 8v battery to other GMC 6v positive ground electrical devices?
