Should I bore a 74 318 to 340 ci or just go with a 360?
Why not? Stroke being equal in a 318 you get 97% of the piston area that a 340 has. That's the only real difference unless you want to talk about heads and breathing. Toss the same aluminum heads on each engine and the same cam, and carb you should be able to develop about 2%-3% more power with the 340. Doesn't really seem worth it. If you do want to use the original X-Heads on a 340 it's up to you. Forty year old technology will guarantee a loss to a 318 with magnums or Eddies.
Besides. I never said this.
What makes you think you can build a 318 to make more power than a 340 for less money?