You can try these guys maybe:
http://www.turnkeyenginesupply.com/streetpackages.html
<<------ And I think Scwartz & Mizfit sponsers sell turn-key setups. Check the sponsers on the left side of the page.
LS6 is virtually the same motor as the LS1: 346 cu in or 5.7L aluminum block. It has better flowing LS6 heads and a slightly more aggressive cam as well as the LS6 intake. You could buy a pullout LS1-T56 for alot cheaper since they came in the F-body (Camaro/Firebird) where the LS6 only came in the C5 Z06 vette or CTS-V Caddy. Add a nice cam and heads setup on the LS1 and you will be well over the power of a stock LS6.
Now alot of people are going LS2 which is a 364 cu in or 6.0L alum. block. Best bang for your buck might be an LS2 or 6.0L iron truck block with the new L92 Heads and L92 intake. I'd look inot that option if I were you.
Otherwise, do alot of reading and searches over on
www.ls1tech.com there is a ton of info on these motors.