There is a REALLY good thread on this, some time ago. I don't remember who posted, but they said something like, "imagine one of your model motorcycles (everyone has at least one of those). lean it, like you would as you're riding it. Then put downward pressure on it, just as you were riding it. You will see how the tire (profile) deforms."
I thought this was an AWESOME demonstration of tire flex.
So, let me clearify... assuming your 400 takes a 120 (front, also assuming your friend has a 120 front), I would use the front, and set the rear aside. The control is in the front, not the rear. If you've got super sticky tires in front, this means you have that much more control in the front.
I hope that cleared (clear as mud?) it up for you.