340cui with .070 bore

340's had a factory bore size of 4.04. When overbored the amount comes off the diameter, bored .070 equals 4.110. Like adamr said sonic checking should be done in order to be sure of wall thickness. I've taken a high compression race engine to +.060 with good results without sonic checking but consider it lucky that it lasted well. A partial fill with block filler may help the cylinder walls a little also.