Not to knock the 340 (Hell I have/just built one) but do yourself a favor. Go to the junkyard/pick a part, and pull a late 80s/early 90s 360 LA block roller motor from a pickup. You'll be steps ahead of the game with the roller already there and you can build the 360 for cheaper. Remember that TORQUE is KING on the street! Plus all the cool bolt on parts for the 340 will bolt on to that 360. Now if you're worry about someone saying its a 360 and not a 340, tell them its a 340 (only why they'll know is if they look at the number on the side of the block) and if they say thats an externally balanced motor and all 340s are internal, call Bullsh!t and clearly state that from April 1st, 1972 ALL 340 motors were externally balanced (and yes, a late model 340 externally balanced motor can be built to be an internally balanced one because that what I did with mine).
That said if you can get the 340 for cheap and is got a stock bore or close (I paid $50 for my 340 block) go for it. But I'd sell the 340 block to someone else and go buy that LA roller block motor. This is just coming from someone who just built a 340 which I can get you specs if you want.