For a streetbound 302, a 1 5/8" primary pipe is ample. If it's a race oriented deal, 1 3/4" or
maybe a 1 7/8" pipe on a 7500+ rpm mill might eek out a few extra hp at the top of the curve.
I built a very mild street-oriented 302 crossram below in 2006 for a customer (pictured with his engine after the final dyno pull), and it made an easy and
honest 377 HP with absolutely no tuning whatsoever using nothing more than a set of Hedman p/n 68270 1 5/8" street headers. (I can't guarantee everything will fit w/o any mods, but these pipes were pretty popular with the gang over on the camaros.net forum.)
Bigger ain't always better, esp. when it comes to headers.
Click
here if you'd like to read about the build. There's a short dyno video clip at the end as well.
Eric