318 Mopar vs. 340 Mopar

I have a 71 dart with a 225 slant 6. I am thinking about putting a v8 in it. After looking at what it would cost to build the 225 I think I might be happier with something bigger. What I am looking at right now is going with a 318 or a 340. Could someone tell me the benefit or difference between the two. Thanks