Many of the old 4cyl IH (M, SM, 400, 450, etc) are + ground.

I charge batteries from them via the starter terminal cable (be careful of gas tank!!!). I put the + (red) cable from charger on the starter. However you had better know for SURE whether you have + or - ground machine. Make a nice label for the tractor noting voltage (could be 6v) & whether + or - ground & put it near the starter. Charging to the battery may be better, but if you're trying to keep the battery box & paint in show condition, then you'll not want to break into the box any more often than needed.