A typical American diesel pickup is a full sized truck.
This class is limited up to and including one (1) ton vehicles.
That would include mini trucks, micro trucks . . .
To answer your question the rule book would have to list every possible combination of vehicle.
Instead they make it generic to say that they want an off-the-lot truck, not a custom built.
The gas coupe and sedan rule references are more about safety, modifications, and other things.
The engine is another matter. You would have to get clarification before you build, but I would think anything diesel you can shoehorn in it would be legal. I would think taking a gasoline engine out would be legal.
Again, if you are going to build, document the plan and submit it to Ed Whiteley the Truck Category/Pickup Classes Chair.