Displacement=size of the motor. and yes, the 4.2 is the right size. its basicly a 5.0 with 2 cylinders lopped off. As for the 5.4L, you would probably have better luck with that motor if you want more OEM power. If you plan to do more modifications, you would have better luck with the 4.6. You best place to locate either motor for your truck with the complete harness and computer would be a scrapyard. if you are lucky enough to find one that has been hit in the rear end, thats a goldmine. As for the 300I6, this may be doable. they were installed till at least 92, and there is an abundance of fuel injected motors in yards. At least locally. Im not sure across the country. The main problem would be checking for clearance. And possibly emissions. Anyway, hope this helps