340 or 440

Seeing that you have one all ready to go in, I would go with the 340 for now and determine if it has enough power for you.
If not, then make plans to build the big block.
Depending on what route you take for the power level of the big block, it can get really expensive really fast.
bigblockdart.com has a lot of great info on there that you can read to help you.

By the way, i am a big block guy, but there is nothing wrong with going small block in your a-body, i did and have plans for a big block when the money tree gets in full bloom!