I'm swapping out the load rated tires on my 1-ton dually for light truck tires. For me, that makes it a 7/8 ton
. I still get the increased stability of the duals w/the heavier duty frame & suspension & a better ride. I have no plans to tug around a 5th wheel or HD trailers so I shouldn't have any issues. You would be doing something similar (but opposite?) & need to swap the HD parts like described. Use the 3/4 or 1-ton front suspension (a-arms, spindles, steering, & brakes) & bolt it up to your existing x-member. The 3/4 ton brake rotors would have a narrower track width but the whole spare-tire thing needs to be considered.
At the minimum, the rear would require swapping in a HD housing, cutting the sides of the Burbs sheetmetal for tire/wheel clearance, custom/fabbed shock mounts (dually rear housings mount the shocks in-board of the frame, 1/2 tons have them mounted on the outside), & the dually fenders. Check w/CaptKaos for a source of OE quality replacement fenders.
How are the fabrication skills?