255's should get you into high 11's with MT street slick IMO. My car had 235/50/15 MT ET's when I got it and the PO took it to the track only one time and made 3 runs with it. The first 2 runs blew the tires up and so he did not run them out and and got an "OK" run on the 3rd try with a lot of tire spin. Ended up running 11.98 at 120 something with 3.70 gear. This was only running the primaries on the carb. The secondaries were not hooked up. I think you'll be mid-high 11's with those tires.