I have a 318 laying around that I wanted to rebuild to a 340. What has to be done to achieve the same specs as an original 340?

 

318 & 340 had the same stroke but the 340 had 4.04 bore, 318 3.91 bore. It can't be bored that much.
You could go with a stroker kit and get as much as 390 CID.

 

So how much can you bore a 318 would be a better question.

 

60 over seems to be the max.

 

Bore it 60 over, put in a set of HC pistons, and good rings, then use a hotter cam and headers and good dual exhaust. Add a good aluminum intake and a Carter or Edelbrock carb and you'll be very pleased at the power. Back when you could still get a 340 in Dusters and such, a guy here had a built up 318 in his Duster. He would slay all the 340 based Dusters save for the one that had basically the same mods he did and that was a close race.