What is the length of your tank? 8'? And what offset do you have XR15's spaced apart from each other? This is the setup I have been eyeing up for my 300G build I have in the works. I also have a eurobrace top but wasnt sure how their mounting arm brackets would attach to the tank. Can you describe how it works?
There are various rail lengths sold, including for 4', 6', 8', etc. tanks. All use the same "legs" which attaches to the rails via screw clamp. You can move the legs forward or backward on the rails to fit exactly to your tank. If you have eurobrace on top, then you likely won't be able to use the clamps that comes with the legs to secure to the tank.
Fortunately for us, if you set the legs from end to end or side to side (using shorter rails between) the set-up will hold just fine. Essentially the bracket on the leg will rest on top edge of the tank forming a 90*. The bracket is wide enough to not tip over and a snug fit against the tank will create stability. I was skeptical before trying it. My lights ~18" inches apart.
I hope that helps.