'67-'69 Barracuda rear tire size

On my '67 Cuda I'm getting ready to yank the old rear. I'm gonna put a 8 1/4 in place of the 7 1/4. I have a pair of steel rim's I had built with a 4 1/2 " BS does anybody know the widest tire I could get in those wheel wells. I would like to go with 255/60/15. If possible, I just don't want to order something that wont fit.

Thank's Larry