ok i want to make about 400 all motor and about 600 on the bottle, should i use a 408 or can i do this with a 347. Ive had every thing from a from 347 to 408 suggested but i am fairly new to the ls1 thing all my expericene comes from turbo hondas... thanks guys the more info the better

 

Yes you can make it with 347, pretty easily on a forged setup.
Now i'm assuming you are talking rwhp, because if it is crank, a stock LS6 is 405

 

Depends how much $$$ you want to spend and you goals. A decent h/c setup should see 430-450+rwhp n/a a 150-200 shot would suffice. However on a stock bottom end many say ~550rwhp is about the limit or you'll be on borrowed time. Forged is a different matter though. A for sure a built 408ci will make a decent amount more power.

 

347,383,402,408 anything forged and your good if you build a stroker bad *** heads are a must!