I found 2 1/4" pipes with 2" tail pipes to be the best compromise on a street driven car. That is if you are going to get rid of the Y-pipe and add duel exhaust using the stock split manifold. Mufflers I don't comment on as sound is a very individual preference.
On the cars I ran headers and a bigger cam on I did run 2 1/2" pipes from the collectors to the mufflers, then 2 1/4" tail pipes ( mostly just short 1 foot pipes after the mufflers. ) That was used on a more 1/4 mile orientated cars, that system caused a loss in low end torque, but the ET's & MPH improved, and the engines rev'd higher. Also nte that the smaller 66/67 230 ci engines rev much higher than the larger 68/69 250 ci versions.
Also just so you know an inline 6 with duel exhaust does not sound like a V8 with duels, For some reason they sound louder to the human ear than they do on a decimal meter. When my local track started enforcing noise restrictions of 80 decibels for a Gasoline powered car ( other fuels had higher limits) My stroked OHC-6 with headers, duel exhaust 2 1/2" pipes, (I forget the muffler type) and short 2 1/4" tail pipes registered 78 DB's, but sounded louder than cars that failed the DB meter test. I think it is the frequency of the cylinders firing, 2 less bangs per engine RPM than a V8. But that is just my theory.
