Marv,
What gauge are your jumper cables? Most sold today are designed to carry 12 volt current, not twice the current necessary for a 6 volt starter. And being 8 foot long doesn't help either.
To add to the above comments, you would need to adjust your 6 volt voltage regulator to a higher voltage to recharge that 8 volt battery. A standard 6v regulator is set at around 7.4 volts and that just won't work with an 8 v battery.
Four of the trucks in my signature line have run for years just fine on 6 volts; only the '42 was already converted to 12 volts when I got it. I have had to once clean ground connections on the '51 because of hot start problems. The trucks are stock so I don't have modern radios or other accessories so I don't need 12 volts. All have heaters and two have original radios.
Kent
