JAS -- A 255-60 is no problem on an 8" rim, even for a 7" rim. My rears are 275-60 .. previously on factory 7" rally rims.. now on 8" rims... no problem. The taller sidewall does allow forgiveness without severe pinching, like on shorter 50-456-40-35 sidewalls would do.
My fender lips are flushed (f/r), car is lowered f/r, ... no rubbing at all.. tight stiff suspension helps prevent diving.
The 8" rims have a slight extra offset of 3/8" (pushing the rim out towards the fenders a bit)... still.. no problems. I really like the taller sidewalls (60) with my stiff suspension setup -- it allows some extra cushioning (for "street)... and an overall comfortable ride.
Like I said, for better performance driving (HSAX/HPDE...road course lapping sessions), that's when I put on the better rims/tire setups... it makes a good difference overall.