Braided strap to the chassis IS ground, to be sure. But on my truck it is the positive, and the negative goes to the starter. Back to that later: First, have you cleaned all your connections yet? Good. Then hook it up with the ground to the negative and try the starter. If the engine turns backwards your system is positive ground. No harm done. Reverse the polarity and try it again. (hint: if you're sitting in the cab with the hood open, peeking through the crack at the fan, and it turns in a counter-clockwise direction so as to pull air through the radiator, things are good)
Probably the best and easiest way to determine if it is a 6V system is to look for stamps on electrical components. Look at the coil. It's right out in front of God and everybody.
I clean my oooooill baaaaath aaaaair cleeeeaner with gasoline, outside of course, then hang it to dry and refill to the line with light oil, like 10W.
