Where do you get this info from? I know many people in the 400-450rwhp range on N/A 331-347 Kits. Buy a cheap rotating assembly. Forget the forged stuff on an N/A build, only thing you should get is forged pistons. These kits are $400-$600 Used, new for $819, and used is not a bad way to go if you inspect the parts. Next, use your block and take it to a machine shop, they will need to do the clearancing for everything and might as well do everything else too. My machine shop charges me $110, for boring of the cylinders, clearance of the rods, bearings etc. He hooks me up by also doing the heads and checking PTV and the rest of the top end stuff. For the price and hassle its easier to drop it off and have him do it. Have i assembled motors before, of course. But i know what is involved and would rather spend that time prepping the car while they make sure everything is in order.
Next, get a custom grind, cams from FTI for example are only $325, thats $25 more than what a shelf cam would cost from anyone else. And thats the thing that will set your build apart from the other 320hp 331's.
Do this and you can have a complete engine for considerably less than what you thought.