Assuming you went with the correct engine, be prepared to spend (if new) about $6000.00 on the complete engine. I think it would swap in fairly simple, you would need to flush the gas tank, change the gas cap (red for diesel) you may have to do some changes to the wiring, and you would need the diesel motor mounts, not sure if the radiator is common between the two, if they are common then you would need the correct hoses for the diesel.
Forget it! I helped swap a 455 engine into a 425 and it was a nightmare! The wiring harness is 1 assy that covers everything from the engine to headlights to taillights. Computer had to be swapped, fuel tank and lines, ignition switch.... I'm sure there is more. Luckily we started with a complete running 455 so we had all the parts, and it needed all of them!