What year 340 block, crank, are you starting with? I'd keep the compression around 10:1 or so, and you will still be able to run on pump gas. What heads do you have for the 340? What is your car, a 4 speed or an auto?
I have a 1969 block bored 40 over, magnifluxed and ready to go. That's it so far, no crank, no heads, just a block. The car is an auto (727) and I'll probably leave it that way. I love the thought of a 4 speed but I'm near 60 and not sure how long the knees would put up with clutch duty.