There's a reason for that. A blower that size is simply not practical and needs to be spun very hard to make the power. There are all kinds of other issues with running a big Whipple or KB. You need to double keyway your crank, you will have issues with intercooler, issues trying to tension the belt and keep from slipping etc. I suggest you do a large amount of research before you jump into the big blower world, it's not as easy as it seems.
That's why I went turbo.