* Engine Management: The 1995 Cummins uses a vastly different engine management system than the 2003. You'll need to either find a way to adapt the older system to the newer truck's wiring harness, which is extremely difficult, or source a compatible aftermarket engine management system, which will likely be expensive and require significant tuning expertise.
* Wiring Harness: The wiring harnesses are completely incompatible. You'll need extensive rewiring or a custom harness.
* Transmission: The transmissions are likely different and may not be compatible. You may need to swap the transmission as well, adding further complexity.
* Mounting Points: While both are Cummins engines, the mounting points and engine accessories might not align perfectly, requiring fabrication and custom mounts.
* Exhaust System: The exhaust system will need to be custom-fabricated.
* Emissions Compliance: Depending on your location, this swap could violate emission regulations.
In short, this is a project best suited for experienced mechanics with extensive fabrication and electrical skills. It's not something a novice should attempt. The cost of parts, labor, and potential unforeseen problems could easily exceed the value of the truck.
If you're looking to upgrade your 2003 Ram 3500's engine, consider a more compatible engine from a later model year Dodge Ram, or a professionally performed engine rebuild or replacement. The difficulty and expense of the 1995 Cummins swap far outweigh the potential benefits for most people.
