Better to have bore thickness for ring seal than boring something out the outer edges of usability. Simply boring one out to a 340 is not going to make it significantly more powerful all by itself I don't think.
Years ago some guy built a 402 out of Poly 318 and it did ok for what it was. I think he made it a 4" bore and used 360 pistons? That article was once easy to locate, but it escapes me at the moment. People have posted it on here numerous times over the years, myself included.
Did more than bore it out to get a 402 out of it. Stroked it too. My math shows a 4" stroke with 4" bore to get a 402.
According to this guy, 450 hp out of a set of ported poly heads is doable. If course he'd building a 390 stroker version.
https://poly318.com/poly-318-cylinder-head-porting/Stock poly heads flow 187 intake, 146 exhaust at .50" lift. No way this head will support lots of HP, stock. I don't care what you chamber is, you still need air and fuel in to make HP out.
Here's a 402 build with lots of head work. 405 hp, 442 tq. A stock magnum 360 with a cam and intake change will almost beat that, 380hp/410 tq. A 360 based 402 with Edlebrocks will beat it. Port those Edelbrock to similar levels and it will destroy it.
https://www.motortrend.com/how-to/mopp-0110-how-to-give-a-318-poly-engine-more-power/ Is it cool to have an off beat engine powering your ride, you bet. I am build a 230 based flathead with hot rod goodies because it's not a belly button engine. But I have realistic expectations of it's output, 150 hp if I am lucky, lol. Is it cost effective, not even close. Sometimes it's about other things than max output. If I was worried about that I'd swap a Hellcrate in.