have you tried it with the campy cassette yet? most bikes run very well, the spacing is so similar. before you splash any cash just try the campy cassette. i ran sram (same spacing as shimano) with a campy cassette for 6 months, faultless. otherwise just buy a shimano freewheel for the wheels, a shimano cassette and freewheel end up costing about the same as just a campy cassette anyway.