How far can you typically overbore a 318?

67 blocks seem to be thin, 68-73 or so seem to be the thickest, have seen a 70 punched to stock 340 bore.

first step to tell if you have a block that can be bored large, is to look in the four sections of the block. See if you can see the casting line, it will lok like a crack almost, running through the centerline from the center of the odd side (I thnk) all the way across the block. :) If it doesn't have this, then you might want to have it sonic tested to see how far you can punch it. I have a 74 block, that couldn't even go .040.