I have seen a few s10 and blazer motors for sale in the area for anywhere from 250 to 500 and I was wondering about the builds of these...

After some small google searches there were a few people who added cams, new lifters, and headers and forged pistons and a little shot of 100hp nitrous to get 400+ easy everyday horsepower....

Are there any recommendations for the build up?

Cam specs?
Pistons?
Bore or not to bore?
Headers?
lifters?
Covers?
Intake?

 

To support that kind of power, you are going to want to get your hands on a marine intake to get conventional injectors for higher fuel demands. I remember reading about the limitation on the stock fuel spider being somewhere around 250-300HP. Now if you are spraying it, you'll be adding more fuel via the nitrous/fuel solenoids so you should be good there, but it would suck to find out your injectors were maxing out on you after you get it together.

Have a read through this article:
The 3/4 350 - 4.3L 262CID v-6 - by David Freiburger courtesy of HotRod.com
It runs through the work that they did to get 300HP out of a carb'd 4.3L.

 

You aint gonna pull 400+ hp out of a 4.3 with out some work. I got a hot cam , a big set of aluminum heads and forged flatties in my tree fidy, and desktop dyno says Im making around 390 at the flywheel.
The first question is do you want to retain your fuel injection?
If not Id run a Hydraulic Roller cam
Punch the block out to wherever it cleans up at (prob .030)
Either Flat tops or .100" domes
def headers but not cheapos cause they wont fit right
Anytime you do a rebuild you should run new lifters ,espeacially with a new cam
Covers? Valve covers? stock ones are fine unless you need you some bling
Intake will depend on whether you retain the fuel injection. Holley makes a good 4bbl intake