Steve Kennedy, Lake Charles, LA, 1965 Plymouth Belvedere 426W

Rick - I just purchased a `65 Belvedere that originally came with a 318 poly engine. I plan to install a 426W or 440 block and heard that I need to change out the K-frame to a big block frame. Is this so? The Chrysler parts book for `65 only lists one part number for the K-frame but I'd think that the engine dimensions are significantly different between the SB and BB engines. Thanks, Steve