[FFR428]

Not all 391's can accept a .080 overbore. You need to check the cyl wallcores. Knock out the freeze plugs and see if the shank from a 13/64" drill bit fits snugly between the wall cores. If it does it's worth the sonic for the 4.13 overbore. If a 17/64" drill bit fits snugly between the wall cores it has the std 390 wall cores and will be hard pressed to go .080. I've seen them both ways. Even if the 428 bore is not doable it's a nice HD block for a build. G.