Largest Rim Tire Combo for 68 Cuda

O.k., I've searched the threads but haven't found an outright answer to my question. What is the largest wheel/tire combo I can go with on a 68 cuda. It has SS springs, not sure if that matters. Right now I have 15 in rims and my BF Goodrich drag radials aren't cutting it with the 408:snakeman: My tire size right now is 235/60 R15. When I looked at it on the rack is seems I have at least an inch and a half of room to spare between the tire and the spring. Just asking what you guys with similar combos are running.

Thanks as Always,
Darren