I've got a little bit of experience with Eaton supercharged vehicles. I've had a few of the blown 3.8L Thunderbird Supercoupes, a Gen II Lightning, and more recently I just sold my 2004 GTO that I had put an Magnacharger Eaton M112 kit on - 455 HP at the wheels.
What engine is he planing to put this supercharger on? Any appetite for making or buying custom parts, like an intake manifold? If he wants a bolt on kit, BDS is another company that makes 6-71 based kits for Carbed engines. I've never dealt with them, but they are very popular, and seem to be reputable.
http://blowerdriveservice.com/blowerkits.php
The newer blower designs are more efficient. Magnusen, that made the Magnacharger kit I used on the GTO has a couple of Carburated kits for around the same price range, but I think only for the Chevy small block, and the Ford flatheads. Like Kenne Bell, and Whipple, they seem to be concentrating more on modern fuel injected engines. The Kenne Bell, and Whipples are Lysholm screw type blowers that can move some more air and make more power than the Eatons, which are basically updated Roots blowers, like the 6-71 or 8-71. However the Eaton is the supercharger used on modern cars like the 635 HP ZR1, 550 HP CTS-V, Mustang Cobras, Lightnings, and the Ford GT. They tend to be more fuel efficient, and behave a bit better for a daily driven vehicle.
http://www.magnacharger.com/
http://www.whipplesuperchargers.com/products.asp?CatID=39&DisplayCat=1
http://www.kennebell.net/KBWebsite/Home_pg/layouts/Homepage.htm
If there's a lot of money to throw around, putting in a newer Modular motor with fuel injection, and running one of the newer kits can make just about any amount of HP he could want. Some of the upgrade kits coming out now for the 2011 Mustang Shelbys are flirting with 1000 HP.
But if he wants crazy power, and is not afraid of spending the time and money for a custom setup, turbo's are the way to go.
I just caught the part about it being a 400. If he is insisting on staying with that engine, he might have to go with a custom intake manifold, I don't think the company you listed has an intake for the 400. Also if he is planning on making much power with a supercharger, he should count on building the short block specifically for it, or else that 400 won't last long.