Mine have body mounted 1025s, so I can't help. One idea would be to cut the shank on your Unimates, drill both pieces, and lengthen them with a piece of brass rod. You could use any of the Unimate lengths, since the rod length can be varied, and and you can set the coupling distance however you like it.
Another option, and probably a lot more work, would be to screw a piece of brass to the truck, in place of the coupler box, and screw one of the body-mount Unimates to that. I actually did that to an N scale European low-deck flatcar to truck mount MT Z couplers for my narrow gauge. It's a pain getting everything at the right height, but the idea does work.