The 4000 series has a MUCH heavier rear end. No, the 3/4 or 1 ton axle parts will not interchange. You will need some sort of heavy duty 5 speed transmission with overdrive on 5th.gear to get more highway speed, or install an auxiliary transmission like a Brownie or Spicer with an OD top gear. Be sure to make upgrades to the brakes and steering to make sure your vehicle safe for the rest of us who have to share the road with you!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!