There was a series of articles in MM&FF a few years back where they built up a 426 inch Windsor with KB blower on it. I think their motor had some compression in it, like 10:1 which obviously isn't optimal for a blower, but the rest of the combo was TFS Hi Ports (aka Street Heats), a fairly big Isky cam (something like 230/240 intake/exhaust durations), a GT40/Lightning lower, and a KB 1500 blower making like 7#'s of boost. I can't remember the flywheel HP, but I want to say it was something like 580 ish, which for 7#'s of boost wasn't that bad. Sorry to blab on like that, but point is, if you could find that article, it would probably help you a lot in deciding if you want to go the KB route.
As a former KB blower owner (on my LX), I can say from experience, they are a fun street blower. Lots of torque, just like they say, and it is a lot of fun to be able to stomp down on the gas pedal, and just blast the tires off the car. And it pulls pretty nice once the tires hook up too.
But..........as a current Vortech owner, I gotta say, the sensations are different, but the Vortech does support more HP. That's why I sold my KB blower, it will only support so much HP (as the kit comes anyway, there are guys that have fabricated other parts, such as inlet manifolds and intercoolers, that will make more HP, but that's a custom only deal, not the way Joe Average buys his KB blower). And the pull off the Vortech as the R's go up is pretty fuggin' cool. I saw a full weight Lightning at the track last year running in the low 11's with a stock bottom end, Edelbrock Peformers, a cam, and a T-trim pumping like 18#'s of boost. I think if I were building a 408, I'd probably want to put a T-Trim, D1, or a Novi on it.
Hope this helps,