Why? 4.04 vs 3.91 ID. That is .13 of an inch. Not worth the expense. 340 power was in the combination of heads, cam, compression, ignition curve, exhaust, and induction. It really is that simple. 340 blocks are a little stronger, but they are both made of good material. No one wants a 318, so just swap the good stuff onto any small block and you have a power house.