this is something you may or may not find useful, but
Primary pipe DIA
(CID / # of Clys) x Peak Torque RPM / 88,200 / Pi x ^2 x 2 + (primary pipe thickness x 2) = desired primary pipe OD
Primary pipe length
http://www.bgsoflex.com/auto.html and this right here predicts a 42.50 inch primary length using an 1-1/2 header (playing around with this also indicates that a header length of 14 inhces and 1.5 dia primaries produce best power at 18,000 rpm
)
Typically peak torque occures 1500 rpm below peak power so on a 302 designed to see peak power at 5500 rpm the primary pipe dia should be in the neighborhood of 1.55 inches in OD.
The length is figured using 6000 as the max revs.
Thats a pretty long header though. And on a street car with a full exhaust, your probably wouldn't get the real benefit from this.
Anywhosit all this stuff boils down to pressure wave tuning which is beyond the realm of the average car crafter in terms of tuning.
Although if the cam was right, the intake was set up for best torque, this thing would produce a sharp spike at 4000 rpm.
What most people end up with is an exhuast system that isn't optimized for thier combo and provides little in the way of tuning, especially with shorty headers. This isn't as bad as it seems since peaky engines aren't that good at producing best average power.
Its ineresting to note also, using the bowling grippo page, that a 347 with a max engine speed of 7000 rpm using an 1-3/4 primary, needs a 30 inch long-tube. which it seems hooker designs thier header length at