A 33X10.5W will hook the car with whatever you want to put in it. I would go with quartermax as the shock mount is fixed to the bracket. If the car is set up properly you do not need adjustment those bold holes get disturbed over time. I would go ahead and move up to a round tube back half. You are already talking turbo. And when doing power adders people ussually move up in power pretty quickly. This way you can go 25.2 certification pretty easy. Might as well build it this way to start with. Call SFI a 25.2 spec will cost you $35 and is well worth it. An you cold run faster than you ever intend to at this time. But trust me when you watch your freinds spend money twice to keep up with certification you will thank me for the advice down the road. The package tray bracing is meaningless when the car is built properly like P body said trianulate. No need to X brace if you run a rear halo bar and tube joined across that makes a neater job. Another thing try to pay extra now and get a 4- link with 1/2 inch bolt holes as this will give you far more adjustment in IC.

Leon