318 into a 340

No you can not bore a 318 to 340 bore size. You may be able to go to 4.000 inch (360 bore size) if the block sonic checks out OK. If the heads have a shaft rocker set up or simply a standard valve cover, then there not poly heads.
If the valve cover has waves or wavy bottom, then it's a poly head. There a canted valve design. The head can be ported to a decent level. It will not port out better than a LA head. The design of the head in other makes is more advanced than the older Poly head and much more potential.
Chevy heads are of this design. There revised ports would have been much more inline with that heads evolution should Chrysler had decided to continue with that head.

The Poly head was also nicked named "Semi Hemi" head. Performance parts for the older poly engines are out there. Headers are very expensive, intakes are used save the Weiand low rise single plane which is made new. Cams can be made new. Racer Brown has a grind list of over 100 cams ready to grind or custom cut for you.

The MoPar engines book has a list of parts the "A" and the :LA: share.