340 in 68 dodge dart 4 door?

340 engines are rare and expensive, they are great engines if you can afford one
360 engines are plentiful and inexpensive, and can easily be built to make more power than a stock 340
318 engines are almost free and can also be built to make stock 340 power levels, but they can not be bored out to 340 displacement, here are some cool things to read

http://www.allpar.com/mopar/318.html

http://www.1962to1965mopar.ornocar.org/ml-pitchercastingheadarticle.html

http://www.allpar.com/mopar/4bbl.html

http://www.allpar.com/mopar/mopar340.html

http://www.moparmusclemagazine.com/techarticles/mopp_0409_318_engine_build/index.html