A bit late, here are my 2cents.
You want power, good for you, go for it, just be gentle with the throttle at first and do not listen to the guys telling 500HP is too much!
First, do a front clip swap, use a mid '70s Camaro or Monte, plenty strong and you get poser steering and brakes, IFS and a sturdy OEM design. You also get V8 engine mounts, which you can use to easily install an LS engine with adapter plates.
Look into junkyard engines, many good deals to be found, with warranty, low miles...
Do not forget the cost of the electronics for your engine, computer reprogramming, harness modification or aftermarket harness...
As much as the LS3 is a good engine, do not get stuck on that particular engine, there is choice!
I like the idea of a low miles used engine, and I see a couple of ways to look at things:
get a truck engine, low miles and put a blower on it, like a Magnacharger kit, quiet, reliable, makes plenty of power...
look into a late model Camaro/Corvette engine, probably more money up front, but several come with a factory blower...
With a used engine, make sure you also get the computer and harness, and you might want to take the trans that came with it too...
Have fun!
