42" headers sound way to long. I think most are in the 28" to 34" range. The length is set for max power at a specific RPM based on cam, heads, intake, bore, stroke ect, ect.
http://mez.co.uk/mezporting/exhaust_length.html
Make a set as best you can but I wouldn't sweet getting the length exact.
I know it can be just internet chatter but most people consider primary diameter more important then length.
http://ls1tech.com/forums/conversion...o-results.html
http://www.hotrod.com/how-to/engine/header-basics/
http://www.speedtalk.com/forum/viewt...hp?f=1&t=26079