If you want a top quality blower I HIGHLY recommend
www.hamptonblowers.com There IS a difference in blowers. I did a lot of research (years worth) before buying my blower.
You have gotten some very good advice so far but, I would like to add a few things.
If you are going to do it you might as well make some power, it's not going to cost much more. Your goal will be VERY easy to reach. Keep the compression in the 8-8.5:1 range and go with good rods and forged pistons. I would also recommend contacting Engle Racing cams
www.englecams.com and having a talk with them about your goals and they will grind you a cam that fits your needs. You will need a cam with a LSA of 112-114 that is really about the only hard and fast rule. You don't want a lot of overlap.
As far as the crank hub or balancer it is a matter of opinion (which you will see a lot of when it comes to blown applications) I chose to go with Hampton and followed his advice (he has been doing it longer than anyone and really knows his stuff). I went with a crank hub. The problem with a balancer on a SBC is that the crank snout is so small (in diameter) that you don't want to hang a lot of weight on it (ie...a balancer) when using a blower. I would also recommend going with an 8mm drive and not a 1/2" pitch, the 8mm will make slightly more power but, the main thing is it will put less stress on the crank snout. As you can see there is a lot to thing about feel free to ask more questions as you venture deeper into this project.
I HIGHLY recommend going with Holley blower carbs as well. You can get away with a standard carb but, if you are going to do it you might as well do it "right" (matter of opinion as well). The guys at
www.allcarbs.com (All State Carbs) are great to work with and I used them and recommend others to them as well (If you go that route tell them Royce sent you).
For headgaskets I would recommend Cometic MLS gaskets (you do not need to O ring the heads with their gaskets and they seal better than copper. Some machine shops are not up on these gaskets yet and might try to tell you that O ringing is needed (IT"S NOT) especially on a street engine making less than 18lbs of boost. If you keep the boost under 10-12lbs I know of some people getting by with standard composite head gaskets (I personally wouldn't try it).
If your GM ignition is an HEI style it might be a tight fit with the blower, you don't have to have an exotic ignition for a mild blown engine, but you do want plenty of fire. If you plan on pushing the limits I would recommend a Crane ignition/boost retard or MSD ignition/boost retard.
I have a pretty detailed blower engine build on my site with lots of pictures if you are interested
http://community.webshots.com/user/camaroman7d
click on the album that says "new engine"
Do your research and form your own opinion.
As you can see there is WAY too much info to post it all here, hopefully this gave you some things to think about and decide if you want to go deeper.
Royce