These old 6's are done by 3000 rpm in stock form. With liberal amounts of money applied, they can be made to spin at higher rpms, and generally they will hold together until 5-6K without concern in stock form. My suggestion would be to gear it correctly for intended use. With a stock engine, if you figure 3000 rpm at 70 mph, and adapt your rear gears and tires to match.
Devin
