Boost is just a number, don't worry about it! Are you dead set on a supercharger? You can buy a used turbo kit for not much more and you don't have to worry about buying headers/midpipe like you would with the supercharger. Plus you wouldn't have to worry about replacing belts, buying expensive pullies to make more boost, etc etc.
I would go around 400-425rwhp with the car and enjoy it. You're smart for wanting to stay away from the FMU, good move...don't even think about using it. Also, don't think about not getting a tune, if you're spending that much money then spend the extra $500 to make sure it runs right. The only mod I'd make would be a aftermarket intake on it, I wouldn't change the throttle body or anything else either...waste of cash. If you already have it then that's cool but right now worry about getting the blower/turbo/fuel mods/tune and you'll be set.
Good luck!! FWIW, my bone stock 5 liter made 357rwhp with 10psi from a SC-trim without a tune. Off the charts rich, probably would of made 375-400rwhp with a tune, it even had 2 1/4" factory catback on it too.
-Edit-
Also do a compression/leak down before installing to make sure the engine is up to par. I would almost suggest you pull the heads off and go ahead and put in good headgaskets/ARP studs when you changed the intake. Wouldn't be much more work at all and the engine would be ready to serve you for a long time if everything else checks out.