The factory trailing arm crossmember in '63-72 Chevys is a stout piece that does its job well on a stock truck. On airbagged trucks or those that are lowered 5 inches or more, the driveshaft becomes dangerously close to rubbing on the top of the hoop. The stock crossmember also hangs down nearly 3 inches past the bottom of the frame. On a severely lowered truck, the exhaust routing also becomes an issue. This is where the driveshaft was currently riding with max air in the airbags.