Since you do have headers, would be easy enough to order the 3" mid pipe with X from Pypes. Your exhaust guy could easily install that into a set of 3" long case ultraflows and then bend up 2.5" tail pipes. This will give you the best of both worlds plus additional comfort you are getting the max benefit from your system. And, your local exhaust guy can do the work. This is exactly what I had in my 73 GTO and the heavy car was turning ETs in the low 11's and still sounded great on the road (maybe a little loud fornsome) At the track, it was the quietest car out there short of someone running a stocker or a new model muscle car.
Or you can go 2.5" the whole way, lose a few HP, and still be very happy with the amount of ridiculous quiet street power you will have
For comparison, my 66 4SPD GTO had a 455 with SD Performance KREs and Stump Puller cam. RA manifolds with 2.5" all the way back through the more restrictive Turbo mufflers. The car still had more power than I could put to the ground, and was definitely more pleasant driving around town if you do not want to wake up the neighbors.
You have good options
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