Instant center has nothing to do with the weight ballance. Yes you want to move as much weight behind the rear axle as you can but I can't tell you if having more weight on the rear tires than the front is good or not, no experience with that.
Instant center is the theoretical point where the rear will lift the front of the car to transfer weight. This is an adjustable point (front to rear, left to right) depending on how you adjust your chassis. Weather that is with caltracs or a four link or a stock Chevy triangulated four link. The idea is to adjust so that the car will transfer the weight quickly. If the instant center is too far forward then the car will fall back down on the front tires because the tourqe of the rear isn't enough to hold it up and the tires will unload and spin. If it is too rearward then you just lift the rear and don't transfer weight. Left or right and the car pulles one way or the other.