Bill I will not support the + 0.030 part of the 4.25" . But I do know the standard 383 BB piston will equal 426 when you start with the 413 block. If you do this you will a engine size that Imperial or Chrysler never had. As we all know this was a Dodge and Plymouth size. Rodger & Gabby Colo Spgs '47 De Soto Custom w/Fluid Drive & passenger door mounted swamp cooler '63 Le Baron w/F-Sweep trunk lid & std A/C '66 Le Baron w/dual A/C & reverbed AM/FM