I had a Vortec on my 97 C5 ... we pinned the crank, but, most imporantly, did a Lingenfelter fuel pump and injectors along with it. With some other mod's (headers, etc), we were turning 515RWHP. It was sick. However, seeing what Hennessy and LPE are doing for approx $11K, with a 3 year warranty, to me, it makes no sense not to give it to them and let them build you a monster. And, it will be worth more with the badging on it. Far less risk means happier driving.
Which ever way you go, you will have fun! Good luck and enjoy!