exhaust, missing, problem

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I have a few questions, first I was told that I could not put 2.5 on exhaust on my 89 GT from the mufflers back. Also is there a 2.5in exhaust (tailpipes) premade by like flowmaster or Mac. Second I put a timing light on my car and I was watching the mark on the crank I reved the engine and the mark didn't seem to move much. I thought the timing should really advance when under a load. this might lead to my final prob The car runs bad when the air outside is cool it will miss and hesitate under no load situation, but when I hit the gass all is well.
Thanks for the help