What's going to support the forward portion extending over the barrel?
My suggestion exactly. I prefer a set screw with a nylon end- I get these on some AR15 receivers to tighten upper to lower play.What I would do is to drill and tap a hole through the forward part of the rail and the bracket, then put a machine screw with a rubber tipped end (or some silicon sealant) through it and tighten it against the barrel and Loc-Tite it when you are happy with it. That front screw could help you level the rail in relation to the receiver.