get a saw a 4x4 and a 6.... maybe 12 pack and dont be shy haha some are a b**** to put in... my truck the old one dam near fell out and i slid the new one in... my buddies pick up it took us 4 hours and a few busted knuckles.
Just take a sawzall and cut the cobra head right next to the turbo outlet. Then unbolt it down under the truck and it should slide right out the bottom. Then you need to make room for the new pipe, you will see the difference between the round 3" downpipe and flat stock downpipe. You would help yourself immensely if you go and get a portapower to maneuver the firewall. If not, a baseball bat or crowbar will do the trick with some elbow grease... Put the top half of your new downpipe down from the top and you will see which section of the firewall needs to be moved. Then move it until you can slide the downpipe in and have a good little bit of clearance.