Since you already have all the parts, it looks to me that it would be possible to modify the 9-speed cassette splines to fit onto the 8-speed hub body. OD of the spline pattern is same for both, the 9-speed pattern had smaller ID so you would need to file/grind away some of the spline until it fits the 8-speed body. No idea or firsthand knowledge as to if the resulting spline would be strong enough for long-term reliability!

Looks as if a 9-speed spline pattern body swap is also possible though campy utilized different size axles and you would need to match the new body to your existing axle size, so beware that not all 9-speed bodies will work.