My take on it:
If you go NA 347 the best you are probably going to get is around 400rwhp. I've seen between 425rwhp and 450rwhp but that is done by guys that do this for a living.
I don't believe in rebuilding 302's or 306's it's too much money that you get nothing for. That still costs $1000 more than just using an explorer shortblock. So IMO, it's 347 or a used explorer engine. A member even made a post recently about why he shouldn't have built a 306.
If you go the power adder route on stock shortblock, do not rely solely on the power adder to meet your goals. The stock parts are too restrictive and it will take too much boost. The more boost you need the more trouble you will have with things like intake and head gaskets. Get some decent heads, gt40x, edelbrock rpm, tw's, afr 165's, a good street cam and decent intake/tb and you are set.
If you really think there is a future dart build, buy the tw's so you can easily have them cnc'ed (something i've said before) by TEA.
In my time, i've seen 450rwhp reached about as many ways as possible.
I stand by the most cost effective reliable way to do it as a hci setup coupled with a
vortech S trim (or whatever model it is now). It's a proven setup and you will have plenty of tech support from people on how to get it setup and running correctly. If that on3 turbo setup goes wrong (and i've only seen it go wrong in person), good luck finding an paying someone to fix it.
My pick if it were me? Get an explorer shortblock, sell off the heads and intake, give it a cam and some TW's and an intake and drive it. Worry about the rest of the engine later while you build up the
suspension, brakes, trans and rear. Because 450rwhp will break your trans, show you how mediocre your brakes are, cause your rear to spin one wheel and let you know your
suspension is sloppy at best.