IF you have the V8 K frame in your 63-65 B body you do not need to change it to swap in a big block. All you need are the correct mount brackets. I have them for $70 plus the ride Mopar 63-65 Dodge Plymouth Factory B Body Big Block 440 383 Motor Mounts | eBay
I realize this is an A body site but I'm posting in both. Thanks for looking!