Another new guy...+ tire/rim question?

I just joined and am real glad to see a site like this! I have a 72 dart swinger I got in March that I'm working on. It has a 360 magnum now but I have a 440 block awaiting a magnaflux and cook and then I plan to work on this winter. I acquired lots of toys at Chryslers of Carlisle this summer that still need installed. I mini tubbed and moved my springs into the frame rails and changed to large BP and narrowed the rear 6" overall. Now it's time for some 15" rims. Could use some insight from anyone on this???!!! Overall, I have 15" of clearance from wheel lip to springs. I planned on about 1/4" to 1/2" space for rubbing. I need 4.5" back space on a 10" wide rim. I'm planning on going with MT ET streets. Questions I have are on the MT tires, if the size is 29x13.5x15....13.5 is the tread width so how much should I add for sidewall to get overall dimension? Was planning on getting 15x10 rims or should I use 15x12's? Any help on this would be greatly appreciated! Can't wait to finally get her out from on the jack stands and then save up some loot for a paint job.