Patrick Glad to have you as a member of our group. If you install a 400 engine ( which did not get issued until 1972 ) you will find that all factors will lessen in power, fuel economy or any other subject. If you obtain a set of standard sized 383 four barreled any year pistons ( 4.25" ) they will fit on your 413 rods and the new engine size will be 426 cubes. Any NAPA or who ever machine shop can over bore the 413 and when finished your Imperial will still move down the high-ways and by-ways around High Desert, Calif. Oh yeah, how did you find your '64; how is it optioned; or any other tidbits ? Rodger 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