Agree with Moper, if you have a 340 and do want the historic value of the year it matches a car go for it, but a 360 is cheaper.
I have a 340 from a 70 car in an A body, is a beast with a capital B. Never a stock big block ran like this.
Either way to do it correct you will spent at least 6-7k when it is all said and done to make 500+ hp.