Should I bore a 74 318 to 340 ci or just go with a 360?

Cheapest power is a 360, zero deck pistons, decent heads, and decent cam. This will be the most power per dollar you can build. Next best choice is a 318 with zero deck pistons, cam, heads; that will make very good power too.

You need to figure out your budget first. Then after that your options can be narrowed down.

If your budget allows putting a 4" crank in either 318 or 360 will give you big block torque.

Don't bore to 340, that will weaken the block and eliminate the possibility of any future rebuilds.

318s are great engines once you get the compression up and put some good heads on them. But the 360 with its bigger bore and bigger stroke has a huge advantage over a 318 right from the start.