There is different stokes for different folks, Personally out of all the 600 yard set ups I have shot at I like the way the St Louis Benchrest club is set up. The steel gong that is approximate sames size as the target. (Its a bit smaller, If I remember right they are 18x18. ((Standard IBS 600 yard target is 21x24)) The gongs are also at the same distance as the targets, so if you are hitting them center you will hit the target center.
One club I shoot at they just started having steel out, and I think it is so much better than clay birds, and I am not the only one because they have both, and hardly any of the birds get shot at. There steel is much smaller (Than St Louis.) it is about 10x10, and they sit on the ground just below the targets. (Just swinging on a simple rebar frame.) the problem is they are a couple of yards ahead of the targets, so I found that if I am shooting midway between the center and the top, it will put me close to center on the target. The other problem is the bullet splatter leaps up onto the targets from time to time. Another club I have shot at, only had clay birds, and they was actually 5 yards behind the targets, so you needed to splash just under the birds, to be close to center. IMO having the sighters close to the same distance as the target makes things fair for everyone. If you don't ask, or know the distance of the sighters, you may have a frustrating day wondering why your targets are the way they are.
I also like St. Louis set up with the 2 targets out at one time ,because is speeds match way up. I would place targets side by side, if you start stacking them you have to make your backstop higher. You also need to place some distance between the targets to keep crossfires, on multiple targets, and others. I would say having a 2 foot gap between steel gong, and target frame and then between both targets, and then having a 4 foot gap between bench numbers targets, would get you in the right area.