While the D100 was offered with a variety of V8s, fitting a significantly larger engine requires considerable fabrication and engineering. You'd need to address:
* Engine Bay Clearance: Larger engines may simply not fit without significant body modifications, possibly including firewall relocation or modifications to the inner fenders.
* Transmission and Drivetrain: The stock transmission and drivetrain components might not be strong enough to handle a much more powerful engine. Upgrades or replacements would be necessary.
* Suspension: A larger, heavier engine will drastically alter the weight distribution, requiring suspension modifications to handle the extra load.
* Exhaust System: A custom exhaust system would be required.
* Wiring and Fuel System: These will need to be adapted to the new engine.
While some people have successfully installed larger engines (e.g., big-block Mopars, even some more modern engines), it's a major undertaking, not a simple bolt-in swap. You're looking at a significant custom build, not just a parts swap. The practical limit depends on your budget, skills, and the level of customization you're willing to accept.
To get a clearer answer, you'd need to specify what you mean by "biggest" (cubic inches/liters, horsepower, torque) and how much modification you're willing to do. Researching forums dedicated to classic Dodge trucks would provide anecdotal evidence of what others have successfully accomplished.
