[ Are you sure the MT 315/60 won't work? Mine are 315/60's and are 30 inches tall and 12.5 tread with 14.5 section width.
how wide of rims are you using? m/t uses a 8.5-11 wide rim and they say the section width is only 12.9 and 11.9 tread. i ran 315's on my old dart... and i assure you that the 345/55/15's are at least 1.5 inches wider than a 315. the dart had the springs moved in to the frame rails and the 315's would rub sometimes. the 345/55/15's fit like a glove in my 69 plymouth...the 315's would look waayyy small compared to them. unless you are running them on a 12-14" rim?? is that even possible??? 
i dont know, the right size/ stance makes or breaks the car in my opinion, and i just want to fill the entire wheelwell, and i just don't want the same size tire on mine that will fit b body w/ stock wheelwells... does any of my rambling make any sense??
