Buy a used Ls1 or Ls2 from that place in Buffalo. They say they have many 5.7 and 6.0 takeouts from GTOs and such. It should be plenty in stock form, or do some head work and get a few more out of it.
I think Cody Addington is running a stock bottom end LS1 with just some ported heads and got like 350 at the tires.