I've got a couple of 80's steel Centurion bikes that I've retrofitted with modern parts.

The upgraded drivetrain might be cheaper if you went with Shimano - I don't know how committed you are to keeping the bike with Campy stuff.

You may as well consider 9- or 10-speed because Campy 8-speed is crazy-expensive nowadays as HillRider noted, and Shimano 9-speed parts aren't much more expensive than Shimano 8-speed parts. Either direction would necessitate a new rear wheel (or at least rear hub + rebuild; a new rear wheel would likely be cheaper anyway) so you may as well go with 9-speed.

If you're already using an 8-speed hub with spacer your frame likely is spread to 130mm rear spacing, or at least can handle it easily since it's steel, and 8, 9 and 10-speed rear hubs are 130mm spacing on road bikes.