How far is your motor set back?

I've never moved a motor back, but it's supposed to be about weight distribution.
If you move a large weight a small amount, it's a big help in traction. I want to experiment, but I want to know what other things it affects. (I.E. clearances, hoses, d-shaft, etc.)
Moving the front and rear axles forward has the same affect. Even moving the rear axle forward on the leaf springs a bit (considering tire clearance) can help.