340ci vs 360ci engine
A lot of people on here are going to tell you to go with a 360 because it will end up being cheaper. Pretty much any part you need well interchange between the two, but a good condition 360 block can normally be picked up for far less than a 340 block.
I currently have a 340 in the shop for my 69 Dart because, while it didn't originally come with a 340, it was a factory option, the 360 wasn't.
I would say look around locally and see what you can pick up. If you aren't in a rush, you can usually get a good deal on a good condition block.