Don,
Why use an 8v rather than a 6v?
There is no need to do this. Clean-up your 6v battery/ground connections/connectors. Make sure your battery cables/braided straps are 2 gauge or smaller gauge. They were often replaced with 4 gauge 12v cables/straps.
Farmers had 8v tractor batteries for their tractors, so they often used them to replace "weak" 6v batteries on farm trucks.
What will that 8v battery cost you?
However, if you are desparate, and if it is free, and it is in good condition . . .
8v will put some strain on 6v bulbs.
