With a good fender roll and our new 17x10 6UL wheels I currently have 255/40 RE71R fitting easily, and have actually fit up to 265 Hoosier slicks with good roll/pull on the old version of this same wheel, 6ULR.
With our original flares I have fit up to 295/30/18 Hoosiers in the back, 285/30 up front on our 2007.
The Super20 flares require you to cut out the metal under, Ron Bauer is likely the guy to ask on that, he used them on his DP racer. I see that he lists the car for sale here with:
2 sets of tires with only Nationals runs on them plus 4 new tires (Hoosier R75A - 23.5x11x16)
1 set of TR Motorsports 17x9 wheels with 275/40/17 Kumho W710 rain tires