if you haven't seen this yet it can be very helpful
http://www.rimsntires.com/specs.jsp
sense i dont have a dart i would suggest a max with of 245 for the rear. best way would be to get the rear in and have your wheels bolted on to see what you can run while leaving about 1.5 inches from the top of the tire to the inner and outer fenders to account for body roll. you seem set on a 25in tire so you could get away with tighter clearances compared to my 27in rears. if you get a sway bar it could be even tighter.