I converted a 9/10 speed Ultradrive compatible hub to 8 speed (see the discussion under Sachs Aris freewheels.) The Ultradrive and the preceding Exadrive cassettes are not interchangeable. The freehub bodies are completely different.
Campy makes the "Kit-03" to address this.
However, they specify that you use a 9 speed, individual cog cassette (such as Veloce) and not a 10 speed. 8, 9, and 10 speed cogs are different thicknesses and the spacer kit is designed to work with the 9 speed cogs to achieve proper spacing.
I can switch rear wheels between the Exadrive 8 speed Athena hub wheel and the converted Ultradrive compatible Bontrager wheel with absolutely no need for even minor derailleur adjustment. The Kit-03 includes a very thin spacer to put behind the large cog as the 9/10 Ultradrive cassette body is "deeper" than the old 8 speed Exadrive.
As to rear hub spacing, it is my understanding that both Campy and that other component maker went to 130mm from 126mm when they converted to 8 speed. My mid-90's vintage Athena is 130mm.