James, the Explorer PCM can run a 351W if a custom distributor(CMP) is made, plus the oil pan. Todd of RPS has done all of the hard work for his. The rub is how much trouble all of a bigger engine is. The custom headers, front dress(if not 302), and PCM etc, that makes a 302 so much better.
You can make any power that you want with a 302 based engine. A recent dyno sheet posted on SBF tech had a 375(ish) NA engine making 600HP at 6000rpm, and 720hp at 7000rpm, peak being near 7200. That's the high end stuff, but with the right parts a 347 can easily make that with decent boost.
The key is having an air charge low enough to reach a good boost level. I don't want a "hot" 6psi supercharger, I want a "warm" 8psi boost, inlet air charge. If that can be done, then 15psi etc. should be at the limit of the engine parts.