I used the 2x3" square tube. Slot open the front part of the rear sub-frame and the square tube slides right in nicely so you can weld it all back together. Only downside to doing it this way is in the back seat feet area the tube comes thru the floor by about 1". It gives a real nice increase in torsional rigidity. Some guys cut the tube to fit the floor's contour, I don't like this because the thinner the tube gets and less rigid it is.