Spindles will not swap. Or at least not without swapping the control arms and balljoints first.
If you want 6 lug on the rear, you're going to have to swap the entire axle assembly. The 8 lug rear has no interchangeable parts with the 6 lug.
How far gone is the 1/2 ton? You may be ahead to restore it rather than the 3/4 ton if 1/2 ton is what you really want.
