The 283 and SM420 are original as is the rear end, I don't know if it's 6.17:1 or the 7.20 but the engine doesn't need a 2 speed rear - plenty of grunt with this set-up, top speed against the governor = 45mph
Your truck would have had the 15,000 pound corporate rear with 7.20 gears as standard equipment and the 45 mph speed would pretty much confirm that's what you have. The 6.17 would give you speeds close to 60 mph.
Mike B
