I have never seen a lowering kit advertised for the 73-87 3/4 ton. I know the rear axel tubes are bigger so the flip kit brackets from the half ton kits won`t fit. I would assume that the spindles are a little beefier on the 3/4 ton so I don`t think they would interchange either, unless you converted all suspension parts to 1/2 ton stuff. Although I have seen a lot of crew cab duellies lowered so someone has to make kits to do it. I know belltech make drop kits for the 88-98 3/4 ton 2WD trucks. I helped my neighbor put one on his 94 2500 ext.cab.and everything was different from the 1500 trucks. So you might want to call them.
Now if you are only looking to go an inch or two you could buy shackles for the rear and try a set of 1/2 ton springs in the front. But my opinion is if you are going to lower something go for the glory and set it on the ground.
Hills hotrods in Lubbuck TX specializes in these 73-87/91 style trucks so you might want to give them a call and see if they could help? hillshotrods.com