Once again and with all due respect to those with differing opinions, I thinking boxing the frame is a great idea!! Sure, the frame flexes offroad but it's basically destroying body mounts and any other items attached rigidly to it as it does so. On a pick-up where the bed is separate, I might agree but on a vehicle like the Blazer, I think all that flexing is only detrimental. By boxing the frame, you force the suspension to do the flexing work (what it was designed for) and allow the engine, trans, and body mounts, etc to attach the components to the frame (which they were meant to do) without undue stresses that eventually lead to cracking. I'm going one step further in my eventual rebuild. I will be building a six point roll cage that will also attach rigidly to the frame, adding to the stiffness of the boxed frame. It's just my opinion, but I think that flexing should be left to suspension components, not body mounts and panels.
As to where to mount lines and such, I see you have one of two choices. Either leave "access windows" in the boxing plate with a reinforcing frame welded around it or do like you suggested and mount everything to the outside of the frame.
Just my $0.02 worth
Brian