Umm, how about a 200 shot of nitrous and not a long life expectancy?
Seriously, a 400hp 350 for less than $1000 is a tall order. If you are planning a full rebuild then the machine work alone is going to eat up a substantial chunk of that. Otherwise you could find a good solid lifter cam and maybe a used set of aftermarket heads then top it off with something like a Performer RPM intake and a good carb.
Your asking a lot for under 1k. You have almost a stock rebuild budget. Just my thoughts. 2 thousand would a little more realistic and that would still be tight.
BUT, budget aside I would say easiest way to get to the 400s is either nitrous or 383 stroker. 383 would be preferred.