I'm not overly concerned with originality so I went from six to twelve volts on two of my old trucks. I'm happy with the results and love the way they fire right up with the faster starter spin.
12 v OR 6 v clean cable and ground strap connections go a long way in getting the starter to turn over correctly. Also with your six volt system make sure a previous owner didn't replace the cables with the later 12 volt ones, they are not of a heavy enough gauge and will cause starter problems in a six volt system.
-mike-
