no 340 came from the factory with 10:1 compression.The factory ratings allowed you build to 10.2 or 10.5:1 to be NHRA legal. An early 340 with stock 71 or 72 cc X heads when new was around 9.6:1 or a little less.
I don't think an iron head, 10.1 : 1 340 would run very good on 91 or even 93 octane.
