I have a 2004 E-450 box truck with a 6.0 diesel. I have had problems since day 1. Is it possible to switch to a 7.3? Are there any forums or such that I can go to to research? Thanks.

Besides fitting the motor itself for clearance top, bottom and side-to-side, adapting motor mounts, plumbing cooling and heater hoses, and insuring that the transmission will bolt up, you also need to look at the differences between the fuel feed and engine controls, etc.
The engine wiring harnesses will likely need swapped or converted along with the engine control module. If I left anything, you'll discover it along the way.
Are you ready for that?