James;

Locate the voltage reference (zener diode, or as they called it: "variode") replace with higher value...but since I don't know if they used a 6V reference or a higher one with some scaling I can't be of more help...hell, it may even be a Germanium diode...this is not a job for the electrically inexperienced or faint of heart!

If you get the wiring diagram of you regulator, and take some voltage measurements, I may be able to help further...

The mod to an 8V system may arguably be a worthwhile upgrade, or maybe a manefestation of CMS (reference to Phil's excellent article in May/June Rolling)...before upgrading to 8V, I'd first assure I was not masking some connection problem by raising the system voltage by two cells...clean, prep with anti-corrosive paste, and tighten every electrical joint in sight! In a 6V system, you have NO voltage to spare to loose on poor connections...use search function to find more I wrote recently...

Good Luck