As far as displacement-to-power ratio goes, you are correct.
However, there is a flaw in your general logic, in that this ratio is not terribly important if you want a powerful/fun truck... it really matters more for fuel economy etc.
For example - how'd you like an engine that gets gets 65 hp per liter? Better, right? Well, take one out of a Mazda Protege (or any othe small car). It gets a whopping 130 hp with only 2.0L.
But I bet you wouldn't enjoy that motor very much, eh?

Now, how about a nice big 5.0 or something? Think that wouldn't be a powerful motor, since it only yeilds 50-55 hp/L?
Hell no. The thing is, what matters in the end for your truck is the total power, not the relative power. It's not like you can actively adjust how much of the CCs you use in order to make the most of that ratio anyway. What's you're talkin gabout is power efficiency... a totally different animal from what *I think* you're looking for here.

Honestly, I think that teh amount of work involved in hibrid-izing a 3.0 and 3.4 is greater than just doing a full swap, and dealing with the smog ramifications directly.
Not bein gfrom CA... I'mnot familiar w/ thsi issue. Is it not legal to change the motor? Or, do you just have to have all teh matching equip for the donor vehicle?