Cassette spacing for all 11-speed systems is as near as makes no difference, or so I've heard.

I've shifted Shimano 10-speed drivetrain on 10s Campy cassette just fine.

I have set up a 9-speed Dura-Ace STI and drive train on a 9-speed Campy cassette. Shifts perfectly every time.

I have not tried Shimano/Campy 8-speed to 8-speed, but I think others have and it can work.

Generally, if the cog spacing is 0.2mm or less between the two, it seems that the jockey wheel's lateral play (up to 1.0mm or so in my experience, perhaps a touch more) more than makes up for the "incorrectness" in absolute cog spacing. If you are still iffy about it, and have a +1-speed chain (use 10-speed chain on a 9-speed system), the thinner chain can, in my experience, give you some more margin. Also in my experience, shift performance does not degrade while doing that.

The best advice I can really give is this: you have all the parts, chains, drivetrain (hopefully built up), and hopefully a bike stand, give it a shot! It's completely free to experiment, and if it works, it will be super cool and give you more options, especially if you have a favorite wheelset.

Best of luck!