I have quite simple exhaust. Its basically 993 headers + 993 cat + 2x2,5" 90 degree SS angles
I used 2,5" valve from raceready performance. Allready broke one last summer, and the warranty did not cover for is.
I didn´t do any exact measurements. I just put the two flanges on the pipe to find the best spot for the valve. ( The fit is really thight as you see in the pics) Then i spot welded the flanges on the pipe, cut it and tested the fitment. When the fitment was ok, it was time for final welding.