I see absolutely no reason to remove the trans crossmember for this type of install. It's unnecessary labor. Cut the downpipe at the bottom just before it bends and heads toward the back of the truck. If you've already loosened the clamp you can grab hold of the downpipe from the bottom of the truck and gently wiggle it loose. It should come out pretty easily. The new one should go in easy as well, but it helps to have someone help you line it up with the turbocharger outlet and put the clamp on.