Time to start calling the big truck junk yards in your area to see what they have.
I have a stock Chevrolet 2-speed rear with a SM420 mated to a 350 sbc with a 2-barrel carb in my '56 8400 running 10.00x22.5 tires and it runs 65 mph all day long on the interstate with no issues...how fast are you trying to go? Brakes aren't any good much faster than that...
Mike B
