If you are referring to
Rear Wheel Horsepower (RWHP), then I'm afraid I pretty much have to agree with Hib on this one. Gee... ya got any idea how much I hate doing that?!!!
My original goal with my GS LT4 was 400 RWHP yet still remain perfectly streetable. I've since decided that is a pretty lofty goal for a n/a engine with stock displacement. My current RWHP is 364 (see
http://www.hashmarks.com/gs633.htm for details). I'm pretty certain that with the addition of long tube headers and a few well chosen tweaks I could manage 375-380, but that's probably the real world limit as a streetable 350. Like Hib sez, for that kind of power and reliability you really need to consider a 383 or 396 stroker.
On the other hand, if you are talking about
flywheel HP, then 400 is a much more readily attainable goal. For example, my 364RWHP roughly translates to 428HP at the flywheel using a conservative 15% correction factor. If my math serves me correctly, you only need about 340RWHP to equate to 400 at the flywheel. And honestly, that's quite a lot of fun on the street.
A lot of folks go with the GM hot cam because its cheap and it works, but you do have a LOT more choices. I originally went with a Comp 210-224 unit because I wanted a lot of torque and a stock idle and that cam delivered. Still hungry, I recently swapped to a Crane 222-230. It has a bit choppier idle (which I like, actually) and winds a lot higher, but I'm still tweaking this combo. Like Hib sez, more cam gives you more (potential) tuning problems, too.
I think the Comp 305 cam would be a nice grind for a street machine. Add to that a set of CNC heads, 52mm throttle body, Corsa or Power Effects cat-back exhaust, adjustable fuel pressure regulator, and a professional dyno tune and ECM reprogram and you should be real close to your goal.
GOOD LUCK and most importantly, HAVE FUN!!!