Have a 396 big block w/ a 750 double pumper, Comp 268 Xtreme solid cam 230/236 duration@.50 and .553/.568 lift. I have dual 3" exaust w/ an x-pipe. Would 2" primary headers be too big or should I go w/ 1 3/4" or 1 7/8" primaries. Do equal length headers really make that much difference?

Here's a site with a header calculator, just log in and click on the calculator:
http://www.headerdesign.com

With your engine combination I would go with the 1 7/8" primary and yes equal lenght can be benefical but it is not the end all, on a street car you can get away with unequal lenght headers. The 1 7/8 primary header will give you some good bottom end torque, which you lost alot of with the 3" exhaust, you really sould consider going back to 2 1/2" exhaust you have a mild cam specs to run big exhaust.

1 3/4" tube headers for sure.Summit sells a great fitting painted pair for $79.95 p/n SUM-G9010 for 68-72 Chevelles.Strip off the "shipping" paint,and paint 'em yourself with some hi-temp paint.Quality,price and fit can't be for that price IMHO graemlins/thumbsup.gif