Yes I'm running 255-60x15 on the back of my Demon with no problem with the 15x7 SBS cragars. Fits fine. Your 255-70's are the same width just a little taller tire (2" larger dia.)
I'm using MT ET street radials that are 255/60/15's on my '72 Duster that are 28 inches tall without any problems. I have 15x10 inch rims with 4 3/4 inch backspace and they are very close to the fender lip and spring.
Go to Tire Rack www.tirerack.com and check how wide the T/A is. Mine are approximately 11 inches wide and thats as large as will possibly fit in my wheel wells.
