You'll need a cobra tank for twin 255's like focus pumps. A good operating 255 will more than support 400+rwhp - no worries there. Don't worry about exhausts too much man. My plans are to keep my stock crappy manifold and 2.5" slp X and maganpacks and have no worries about hitting my goals for my DD which are substantially higher than what you're aiming at. Just get some nice LT headers if they are legal where you are and run what ever you want after that. 2.5" would be fine.
You need to pick up some 39 or 42lb injectors as well.
And don't worry about longevity. Your stock shortblock could handle 400rwhp. The one you have now looks to be good to 8-900 all day.