Sorry, didn't realize you were the same person in the thread I posted. Again, I think the C4 sits too high in the rear. Even ones that I have seen in person at car shows.
Here is another example of a truck with the rear frame step notched and installed higher:
http://www.trucktrend.com/cool-truck...-chevy-pickup/
Another build thread with a step notched rear:
http://67-72chevytrucks.com/vboard/s...d.php?t=527123
And another example with a "z" frame and shockwaves:
http://67-72chevytrucks.com/vboard/s...=535686&page=4
The one I posted previously without a stepped rear:
https://www.trifive.com/forums/showthread.php?t=97044
If you're going to pick up the kit directly from flatout, maybe they can give you advice on mounting the rear to lower the stance.
I don't have the C4 in the front. But from the pictures I have seen, it sits at a good ride height. I wouldn't raise the front.
I know you don't want to cut the frame. Maybe you could install the front crossmember to raise the front, but then it would sit really high.