The combustion chamber on the 3.0 is larger in diamer than the cylinder bore of the 2.5 so you would have a "mushroom" above the cylinder. I'm also sure that it would create cylinder sealing problems. The larger chamber volume of the 3.0 head would cause a significant drop in compression. The valves are bigger in the 3.0, as are the ports so you would at the very least need to use both heads from the 3.0 to stay in balance from one side to the other. The early 3.0 heads do not have a water pump drive on the back of the front head. The late 3.0 heads that do have the water pump drive have different port shape that is not compatable with the 2.5 intake manifolding without extensive rework if at all.
If that is not enough to make you rethink the project then I'm sure that someone more knowledgable than me will toss in their $.02 as well.
Basically, there is no real good reason to use the 3.0 head on a 2.5 and there would be a lot of things to overcome if you decide you want to do it anyway.
I'm sure that there are more reasons, but
