to answer your question...it looks like they are 17x8, and according to tirerack, you could probably run a 275/40/17 out back. they may bulge a little but it should work. someone will chime in that is running them.
they are 17x8 and stock they came with 255/45/17 i just looked it up on the svt website just go to the cobra section and compare the years it has teh tire and wheel specs
You can run a 275 BFG DR in the rear, they will fit and LOOK mean.
Now everyone will chime in and say that is the wrong size and too wide for your rim and what not.