I have a 2001 dodge 2500 and I am wondering if I can put the cab fenders and box off a 2003 5.7 hemi onto my diesel

 

If the question is just CAN you, then sure it CAN be done.
But my questions is WHY?

It would have different cab mounts, wheelbase lengths, steering column, electrical. Every turn and every bolt would fight you.
Then if and when you are all done would you register it as a '03? If you did it would likely need to pass emissions standards for a '03...which would be near impossible with a '01 motor.
It would be just as easy to drop a Mazda RX-7 body on your old Dodge.

Unless you want a really hard project, I would strongly recommend finding a better body donor...Like ANY '94-'02 2500/3500 Dodge or even a '94-'01 1500 would be easier.