boost levels and hp levels

ive read in countless posts that the most you want to run on a stock bottom end is 8-9psi and likewise, ive read countless posts saying the max the stock bottom end and block can handle is 500rwhp. which one is right? because in theory you can take a bone stock 5.0 and put 15lbs of boost on it and you wont hit 500rwhp.
im at the point where im making 403rwhp on 8lbs of boost. id like to run a couple more pounds of boost to get a firm grip on the 400hp mark. by that i mean, since hp can fluxuate with heat (i.e cold air more power, hot air less power) all it would take to cause me to run less then 400hp is a worm day.
i have a wide band o2 and adjustable fuel pressure reg so its not an issue of can i tune it but more of which "rule of thumb" should i follow.

well i guess ill put the smaller pulley on and go for broke. i plan to run it as is on the 12 of november and see what it does. ill be sure to make plenty of "runs" down the freeway with the wide band recording to make sure im safe.
i sure as hell dont want to loose my motor but if it did go, id love to build a 331 with a stronger block and shoot for 600hp. as it is, i still have the stock cam in my car the smooth idle is nice for a DD. but being broke as f**k i really cant afford to loose my block or trans for the time being.