Kevin,
Re the use of 1x12 pine boards - this is a bit late, but hopefully not too late.
Wide boards like that, especially the stuff we get these days, are prone to seasonal movement and warping. While I understand your reasons for not using plywood, I suggest that you let the boards acclimate for awhile and then prime and paint them before permanent installation. Let them fully cure before attaching anything.
Also, depending on where you buy your pine, you can look for "select" boards - generally flatter and have tighter grain so they're smoother and more stable. They're considerably more expensive, but IMO a good investment when one considers the amount of time and effort involved in building the layout.
