GT40 crate motors are a waste of time. They sell for like 3 grand and make 340 crank hp which is around 290whp maybe a bit more. For that amount of money you can get all kinds of stuff:
nice block, decent heads and boost
or
Forged 347 storker kit with nitrous
and the list goes on and on.
306 would be more streetable but wouldn't make nearly the power of a 347 or 331; it lacks the stroke. On the other hand, a high compression, race built 306 can make insane power, but wouldn't be very streetable. If you have the cash for a 351, that would be the cheaper route since you can find parts for them quite easily.
Idealy for a KB setup, if you're low on cash, bore out the cylinders 0.30 over for a 306 or 0.40 over for a 308 and buy forged crank, rods, and bigger pistons. Then you can get your blower. This power would be limited to low boost though due to the stock block not being able to take constant abuse from power like that so if you have the extra money i would:
Search the forums for a Dart block or a CSP bullet block, or an R block; something like that. Then if you have a nice block, you might as well buy the large displacement hardware like a 347 or 357 forged stroker kit and build it. Worry about the blower later.
Remember all this power you're wanting is not going to go over well with the stock transmission, fuel, ignition, and drivetrain parts. You're going to have to do many other mods to accomodate that kind of power.