Now that I've had a few minutes to think about it, I'll try to answer your question.

What I would do is assemble one side with backing plate and allow room for an e-brake cable and/or brake line routing. I'd also make sure that the coilover will have room to function without interfering with brake drum/ rotor/ caliper, whichever you're going to use. In other words, I'd make the mounting points on the housing as wide as possible without any mechanical interference taking place. If this allows you to place the housing brackets on the 3" part of the housing, fine. If not, then tell the manufacturer the o.d. of the housing where the brackets will be placed and the center-center of the brackets so he can coordinate the same spacing on the crossmember for the front mounts.