Search should have all the info you need.
If you desire to keep the stock extension and stock cooler and go '09 setup on an '08 then you need to buy the end plate, bumper, gussets (foot peg things), flap, led light, screws for led light, rivets, hose clamps.
The cooler HAS to move forward because the tunnel gets shorter. It's up to you if you want it flattened or not. Leave it curved and it can hold more ice and junk in there, can be seen from the side, and possibly could get hung up on something easier (in theory anyways). Flatten it in something elegant like a press or I did not have a press and merely placed it between two pieces of 2X10 and beat it with a sledge hammer. It's pretty soft aluminum and breaking it I believe is not a concern unless you got really rough on it more than beating it with a sledge hammer.
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If you saw mine before and after you would never know I did anything to it (other than it's straight now). You will have to figure out something to mount it in the back. I found conduit clamps to work perfectly and even have 3/16" holes in them same as all the rivets you are using.
You have to cut enough off the extension to make it all fit.
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Seriously if you rivet the bottom two rivets of the bumper to the end plate and have the two rivets drilled out (one each side) where the front most holes on the bumper are going to attach to the tunnel then it's merely a matter of fitting it than really a measurement. I had the two rivets in the end plate to bumper so everything was set there and just cut enough off that I could even get the thing where it needed to be and marked there and cut. There is no need to guess or really "measure". In total probably 3" or so comes off.
I used a grinder with cutoff wheel for the big cutting and a dremel with cutoff wheel and finally the sanding disk for the more delicate cutting and deburring.