The original 20" tires on your truck are 275/60/20 which translates to :
33" diameter, 10.8" wide.
285/65/20 would be 34.6" high, 11.2" wide
295/65/20 would be 35.1" high, 11.6" wide
If your 2500 is 4x4 you might be able to fit both tires, but might need spacers to bring the tires out a little for a full side to side steering without hitting the frame. In most cases 35" fit under a 4x4, but you would be safer with a level kit which slightly raises the front.
If it is a 2WD, you will need a full lift kit as the body would sit too low and the tires would rub the fenders in the front when turning. Don't want to ruin the new shoes fresh out of the store...
