Unfortunately, this will not fit the stock exhaust, it will only fit the FMF TI4 muffler. The stock exhaust should already be spark arrested on your 2007 Arctic Cat 500 Automatic. If you need replacement parts for this specific model, please contact an authorized Arctic Cat dealer.
We were able to get an answer from FMF Racing directly for you and here is what they came up with:
"The end cap is round measuring 3-3/4 inches across and the outlet is 2 inches across. The spacing between the holes is 2-3/4 inches from center of the hole.
-FMF Customer Support"