I'm in the same boat you are, I'm switching my car from drag-mode to a better overall street car. I have 16x8 rims ordered and ended up going with tire sizes that match the GNX, 245/50 in front and 255/50 in the rear. While it's a huge step down from the 325/50 drag radials I'm running now, I'm confident it will have decent traction. Maybe in your case step up to a 275 in the rear since your rim is a little wider. Just remember the bigger you go the more you have to worry about tire rub.
Go with realistic tire sizes based on what you plan to do with the car. Don't need huge/heavy slicks out back if you aren't worried about launching the car from a dead stop. If you're in it for good handling, then don't bog it down with unsprung weight, something like 255 will be good.
