You would have to change the motor mounts (the ones mounted to the frame, not the engine) for starters. Probably riveted in also so you'll need a torch. Just heat up the heads red hot and use an airchisle with a flat blade to shave off the head and a punch out what the rest. Possibly the radiator also. As far as the transmission, is it a manual or automatic? If it's a manual just keep the bellhousing (if it is the bolt on type (most of the older ones are)) and clutch assembly from the donor vehicle. I did a simular swap a couple years ago with an 81. I took out the 300 and put in a 351m with a manual trans. Good luck.