Having built a couple hundred LT1/LS1 engines I can give you some advice here. The LT1 packages today that we are producing make roughly 430rwhp in 383ci N/A form. To put a blower or nitrous on it to hit 600rwhp is pretty easy, but I think defeats the purpose of the project and not worth the investment into an engine that was given to you. At least as far as I understand things. IF you wanted a turbo/blower you could just build a forged 355 short block and leave top half stock with some upgraded valve train goodies and you'd easily make 500rwhp.
The stock components are weak and are a crap shoot plain and simple when you get over 350rwhp. They weren't designed for that level of abuse, and have been known to give way almost without warning. I compare it to 10 bolt rear ends, some folks have them last a few years with 500rwhp then some cat snaps his with 300rwhp.
A good looking 355 or 383 with a nice cam and such on it would work out well. It would make some good power, have a nice idle note to it, and not set you back too much cabbage. Just my .02
Heres an image of an LT1 done up that looks pretty sharp in the engine bay. Them canton valve covers are nice lookin.
http://victoryracingengines.com/gall...Gibbs/000_0463