If you want a heavy duty starter, get one off of a tractor. I think mine's off of a john deere 4020. I used parts from a '51 caddy starter to convert it to 12V. It has the intermediate bushing, four field coils, and heavy duty housing. Find a good automotive electric rebuilder who can tell you what other starters fit your engine.
But, unless you are starting your truck in the dead of winter, the 6V will be plenty strong.
