You want 400hp from a n/a combo it'll be easier to do with a 331, but it can still be done with a 306. A friend of mine just had a 332 built that on paper is 515hp at the flywheel.
Dropped the motor in last weekend, he's just waiting for the carb.
Oh and btw, you may get 400 reliable hp out of either one, but the question becomes how reliable is the stock block? If I were gonna do it and had the cash I'd go with an R block or at least a Mexican Block. Just my .02