another exhaust sizing question

[65 drop top]

I just swapped out a 500hp 427 BBC for a 600hp 502 BBC. Also did a trans swap to o/d. The old exhaust system is Sanderson 1 7/8 shorty headers and 2 1/2 custom stainless system. The system doesn't fit anymore. I plan on reusing the headers, but is the 2 1/2 just too small and do I need to go to 3? Or should I just mod the current system to fit? It is not a race car so Im not worried about every last hp, but I also don't want it all choked up and not running decent.