318/340 questions please help!!

So heres my question whats the difference between a 318 and a 340? It cant be that much right? Cause there both small blocks and very close to the same look. And heres the other question, Could I take a 318 and make it into a 340? If so how?
thanks

to answer your question
318= plentiful and cheap
340= rare and expensive
http://www.moparmusclemagazine.com/techarticles/mopp_0409_318_engine_build/index.html