318 to stock 340 power level questions.

So, you want to bump your '68 318 up from its stock ~230 hp to 275 hp (stock for '68 340)?

Other than the displacement, the 340 runs a 4v manifold and carb, hotter cam, and more compression.

Assuming you want to run the stock pistons and not rebuild the entire engine, I'd have the heads milled to up your compression ratio a little and up the size of the valves to 1.88/1.60. Go with a 4v manifold and carb. Add a hotter cam with a more modern grind and you should be close.