Well, first I've got to say that I should have proof read better; with a +ve HV the center is +ve and the positive ions are attracted to the grounded outer terminal where most of the erosion will occur.
However, I think my reading comprehension is OK, and "should be wired Positive to ground unless you have an Alternator fitted." (noting the parts following "should" and "unless") is what lead me to believe John was conveying the way it "should" be. And since the poster said it was a 6V system "with a new rebuild generator", John's statement then reduces to the meaning that it "should be wired Positive to ground" and all I am saying is that polarity isn't a significant issue in the least - especially if it was previously running.
And the original post didn't distinguish between slow cranking and failure or weakness to "hit" so we have all made a large heap without getting clarification of the actual issue at hand. His statement that it can be jump started from 12V truck (assumption on the 12V thing) suggests contact resistance for something electrical which is overcome by the assisting higher voltage of the truck jump; with 6V systems contact resistance is just a much bigger issue and yet, properly maintained, a 6V system works just fine.