Ken;

In general terms, in a 6V system, clean and tight connections are even more critical, because one can hardly afford to loose any of the lower voltage to begin with...it must be assured that there are no undue voltage drops alone the Starting Current Path!

I agree with other poster... years ago 6V systems were typical and worked just fine for thousands of vehicles*. Installing an 8V battery won't damage anything per se, but it is a probably a band-aid (instead of correcting the Root Cause: A Voltage Drop in the high current path). You need to Find and Correct this, then, a good 6V Battery in a good state of charge should be just fine for starting vehicle.

What installing an 8V battery does, is give you a higher voltage at the source to compensate for a drop along the way such that even with the drop, you still get a decent voltage at the starter. The better fix (preventative maintenance really) is to disconnect ALL connections involed** in the high starting current path, inspect and clean and reinstall with ACZP***...but this is a "Shotgun Method", of improving every connection in the hopes that along the way, you will also happen upon the poor one which is giving a Voltage Drop and eliminating this...

If you have a Voltmeter, you can also use the "Intelligent Man's Method", and locate just the particular problem connection at which a Voltage Drop is occurring (under Starting Current Load) and fix and minimize that.

Locate by connecting the VM across the Solenoid input power bolt (battery cable connection) and Starter housing, and monitor voltage while cranking Starter...compare this voltage to Battery voltage (measured on Bat terminals, NOT clamps, also under Starting Load)...to determine how much is being dropped alone the way...the Battery voltage of a fully charged (6V) Battery in good condition will likely drop a volt or so, but if you loose more than another half volt or so on the way to the solenoid, you can see that voltage available to drive current through the Starter is already down to 4V...it may then loose a EVEN MORE across the Solenoid High Current Contact (HCC)****, especially if this is in poor shape and heavily carbonized...slow cranking, or Solenoid chatter WILL result...

* as the current is double that of a 12V system (for a given power load), the wire gauge is typically double that which you might be used to on a 12V system.

** Battery Clamps and Cables, Chassis to Engine Strap (if you can't find a satisfactory gauge premade cable, try making your own with welding cable and separate clamps...a gauge where the conductor is the diameter of your pinky should be fine...welding cable also has zillions of tiny strands, so doesn't mind vibration or bends, and although expensive, you don't need a lot!)

*** Anti-Corrosive Zinc Paste see also: http://www.sw-em.com/anti_corrosive_paste.htm

**** Check Voltage Drop across HCC under Starting Current Load! See: http://www.sw-em.com/starter.htm#Stater%20Soleoind measure voltage across Solenoid input Bolt and hop-over cable (no. 5) while under Cranking current load! If VD is excessive (>.2V), disassembly of Solenoid and refurb is in order.

...hope this helps!

Good Hunting!