One problem with that is that the wheel studs are actually mounted in the hub, not the drum. So the hub would also have to be modified. On top of that, using 1/2 ton brakes on the rear and 3/4 on the front may cause a severe brake imbalance, especially if it still has the single circuit master cylinder.
Is there a reason you've put a 1/2 ton rear in it? Or was it in the truck when you got it? I would suggest either putting a 8 lug 3/4 ton rear end in it, possibly from a later truck if a better gear ration is desired, or swaping out the front spindles and balljoints to 1/2 ton components. That way, everything matches.
