There is a huge difference in a 2.78 to 3.36. My car came to life when I changed the rear to a 3.42. Nothing else changed and the car really woke up. I am now redoing the engine and body, so it will perk up a lot more with the higher compression I went with.

You should change the rear after you decide how the engine responds after rebuild. Then you can tell the tangible difference with each change. You may decide you don't need a lower gear depending on your needs.