how to replace a muffler on a 2001 jeep grand cherokee?

i am replacing the stock muffler with a flowmaster super 40 with 2.25 inch in and outlets. The old muffler is about 24 inches long and the new one is only 14, what can i get for the made up length. i was looking at tail pipe extentions that were 2.5 inches in diameter. The tail pipe is 2 inches in diameter. i dont want to replace the tail pipe its self. With the new piping do i want it to go inside the mufflers out/inlets or on the outside of the out/inlets on the muffler?

Tail pipe extention:http://www.autozone.com/autozone/accessories/3A-Racing-2-1-2-in-i-d-x-18-in-length-tail-pipe-extension/_/N-257z?counter=1&filterByKeyWord=tail+pipe+extension&fromString=search&itemIdentifier=402370_0_0_

Muffler:http://performancepeddler.com/detail.asp?PRODUCT_ID=FLO952447