Almost all 6 volt cranking difficulties can be traced to two problems. The first is too-small conductors and the second is poor connections. I got my education on 6V cranking in the early 1960's, keeping a flathead Ford V8 with 11:1 compression running. The same things I did back then work today. Buy or make battery cables from cable at least #1 size wire, and 0 or 00 is better. I use #0 welding cable and crimp, then solder the connectors onto both ends. Run the ground cable all the way to one of the bellhousing bolts, not to the frame or body like it was done originally.
Next- - - - -disconnect and clean EVERY connection in the circuit- - - - -both battery terminals, the ring terminals at the ends of the cables, and be sure the starter housing to bellhousing contact is clean, oil-free, and tight. Get everything clean and shiny with a wire brush or a wire wheel in a drill motor, then tighten the connection securely.
A 500 CCA 6 volt battery is pretty weak. You're sending a boy to do a man's job. Measure the voltage at the battery posts during cranking. If it goes below 4.5 volts after you've done all of the above chores, get a bigger battery. The shock you're getting with the battery charger running is a design flaw or a short to 115 volt house current inside the charger. That's a potentially deadly situation, as it only takes 1/10 amp of current across a major organ such as your heart to kill you graveyard dead! A 15 amp wall plug has 150 times the current capacity needed to provide a fatal shock. If you're going to continue to use that charger (I wouldn't!) drive a steel rod a couple of feet into the ground and attach a jumper cable from the vehicle to the grounding rod.
Jerry
