In my 67 impala, i currently have the single factory exhaust and am swapping the 327 for a 400 would duel 2.5 inch exhaust be good for that engine or should i go bigger? Also, were can i get a set of pipes for my year with no mufflers so i can put my own choice of muffler on them?
The factory used 2 inch exhaust pipe on all of it's sixes and small V-8's. High horse small blocks had dual 2-1/4 inch exhaust pipe, and big blocks and SS cars got the dual exhaust with 2-1/2 inch pipes with resonators after the muflers with chromed tips.
If you want it to be louder than that (factory stock) you can go with a 3-1/2 inch exhaust or a custom oval 4 inch pipe. Diesel trucks all use shiney chromed six inch pipes but the exhaust also points straight up for a reason, as it is barely muffled at all with that size pipe.
A 2.5 in dual system should be plenty good. If you are not concerned about stock appearance, you can add a cross over pipe or have an X pipe designed in to the new system. The stock manifolds will probably become your limitation.
Not sure if your still lookin for exhaust or not. I had really good luck using "Pypes" brand X pipe on my 67. Was cheap, fit really well, worked perfect on my long tube Hookers, and went far enough back that all i did was throw on some SLP loudmouth 2s and turn downs and it dumps right at the rear axle. Everything tucks up nicely, and Im using the 3" set up due to my big block. A 2.5" would be perfect on your car!! Great deal and no custom fab needed. They also offer a complete system if you want tailpipes too http://www.pypesexhaust.com/sys-gmb.html. As for mufflers, I highly recomend using "SLP" mufflers! The Loudmouth 2s flow like crazy and have a deep, unique classic "chambered style" sound rather then flowmasters like everyone else, and none of the Loudmouth 1s drone.
Im a diehard Classic GM guy. Nothing beats the beauty, power, and style of great american engineering!! '67 Impala w/ 502 BBC, 505h.p.