Where you have it mounted will work fine but on e-bodies I always use mounts that use the rear spring hanger hole. Leave it, you'll be fine.

To balance the car:

Put a spirit level on your floor - take a reading. Put the level on the door sill and adjust the car to match to the floor (close is good). This is done with the threaded rod on each end - adjust one up or down as necessary. The crossbars at either end will not necessarily be at the same height since the front and rear mounting points on the car are likely not at the same height.

So now that its level, you want to balance it.

Pull the locking pin out of the sprocket an loosen the lock bolts so the car can spin. Hopefully you haven't already got it too high but that's unlikely). Try to spin the car - push it a foot or so and then let go. If it wants to drop back to its original position then its too low. Adjust the threaded rods at each end equally to raise the car. Repeat trying to spin the car and adjusting till the car doesn't want to drop back to its original position. It should also get easier to spin as it gets closer to balanced.

Depending on what you still have on the car you may not get it balanced at all - i.e. if the rear end and front suspension are still in place then forget it, there is just too much bottom end weight.