the 255/60-15 is 27" tall
how fast you can run with them is determined by your hp ,your cams power band , and your safe rpm limit
your trans and rear gearing , tire height and converter efficeincy will affect rpm at traps, if you use overdrive you can get away with a shorter tire
i can get 1.5s and have run out at almost 120 with some rpm room to spare with my combo in my 87 ( 3.42 left in D), if put those 27's on my 86 the rpms would exceed my motor as i'm crossing the traps at around 6k on a 28" tire , the 27s would raise my rpms about 4% or another 240rpm
whats your combo ? if you dont have the power to go more than 120 id say youd be fine with them