While thinking about it, and worrying that the Constitution and Federation classes didn't convey the proper MAD SCIENTIST feel for the Federation that I wanted to really offer, along with the cookie-cutter nature of the Franz Joseph Star Fleet Technical Manual ships, I came up with an alternative idea.
Class I Cruiser, Destroyer, Scout, Tug Project
Class I Primary Hull/Saucer Section: 35,000 ton DropShip
Class I Drive Module/Nacelle: 60,000 tons
Class I Secondary Hull: 35,000 tons
Class I Destroyer/Scout: 1 Primary Hull + 1 Drive Module = 95,000 tons
Class I Tug: 1 Primary Hull w/ Tug Adaptation + 2 Drive Module = 155,000 tons
Class I Cruiser: 1 Primary Hull + 1 Secondary Hull + 2 Drive Modules = 190,000 tons
Class I Dreadnought: 1 Primary Hull + 1 Secondary Hull + 3 Drive Modules = 250,000
Leaves the Tug a bit overweight and the Dreadnought a bit underweight, but still works. Obviously they're not two separate drives, but sets the mass of your KF drive which, once you factor in overall mass, determines your jump distance. I'm waffling between treating the saucers as Aerodyne DropShips, which would let them fly in atmosphere similar to how the NX-01 does, or treating them as Spheroid DropShips, which have better weapons arcs.