.60 overbore on a 1968 340

Sonic check should be done at that overbore. I've checked blocks that at std bore were thinner than blocks that were .060 over. There is no rule when it comes to these things. Some guys get lucky and just bore 'em 60 and have no problems, while another guy does the same thing and comes back 6 months later with a broken out cylinder. I own 3 340 blocks and not one of them would be suitable for a .060 over bore.