Sorry for the late reply, but I just returned from the Bristol event.
Only FFW knows why we're only allowed shorty headers, but it goes back to the days when the stock in Trophy Stock meant stock! Nowadays we're better off naming the class 'far from stock', but the shorty headers, factory heads and no port work are a few of the leftovers from the earlier days.
Most guys seem to be running the FRPP P specific headers. A few are running equal length shorties, but they're in the minority. Most also seem to have switched to the X-pipes and some version of direct flow through mufflers like the Dynomax Race Magnum Bullets. Flowmaster 2-chambers are also pretty numerous. Tail pipes are out, dumps are the norm. The exhaust system is just one of the keys, so study hard and try to learn from others in front of you.
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Jeff Chambers
T/S #3
11.462 Seconds/117.43 MPH
Chambers Racing Team