I've got a good header design program (Pipemax) that recommends pipe sizes and lengths etc. but here's a link to a free program that seems to come up with similar numbers.
http://www.headerdesign.com/extras/design.asp
You have to sign in to use and it almost sounds like there could be a fee involved but there isn't, I've used it several times in the past.
They say that collector length is one of the most important things on a set of headers.
I find that these programs will usually recommend something in the vicinity of 18" to 20" for cars like ours. 18" might be about right for a car that shifts at about 6700 and 20" would probably be about right for a car that shifts at about 5800
From what I've read the exhaust wave tuning is a bit of a game of snakes and ladders, a length that boosts power at one rpm will hurt power at another rpm. The trick is to use a collector length that boosts power in the rpm range you're using and only hurts the power at some point above and below your desired power band.
The recommended collector lengths in these programs in measured from where the primaries end to the open end of the collector pipe so it normally only applies to open (uncorked) headers. Apparently you can get a similar effect with a full exhaust system if the collector dumps into a fairly large chamber that simulates the open atmosphere. That kind of setup also requires a low restriction exhaust system.
I don't think it's that important to get the collector length perfect but it's nice to get it close, like within a couple of inches.
