The Mustang II kits are all designed for 1/2-ton trucks, so will require modification to work on your 3/4-ton.
I believe that both are the same up front. The rear spring setup is the the main difference. 3/4 ton springs sit on the outside of the frame rails while the 1/2 tons mounted under the frame. I've seen several guys with 3/4 tons use the MII. Check with Big Chevy 3600. He's running the Mustang II on his 3600.
I used the ifs from a '79 1/2 ton myself but there are other choices available. Scott Danforth listed them earlier in this forum.
