Tire Tub size limit

255/70 is going to rub the fender wells as it is slightly to tall

255/60 is right size for a better fit

For a spare I wouldn't worry about getting an exact match.  I'd go with the same diameter (height) as your new tires but go with the stock (for your year) tread width.  That way it will fit in the tub without any problem.

If you have a flat on either front tire you just need to be careful with mis-matched tires on the front when braking, for the most part. 

But if one of the rear tires goes flat it would be best to first put the spare on one of the front tires and then put that tire in place of the flat on the rear.  Keeping matched tires on the rear saves some wear-n-tear on the differential.  An extra tire change is worth not messing up that...!!!