drive shaft rubs on mufler now

Just install my 8 3/4,took out the 7 1/4 had room between the drive shaft and mufflers befor not much tho,but not now.The car is on stands rear hanging down,do you think when I set it on the ground will that help in the clearence.Anyone run into this problim going from one rear to anouther on a 69 Dart.