[Adger Smith]

That depends on what you want the weight to do for your car.
1. work like a lever around the rear end to lift the frontend for weight transfer
2. be static weight over the rearend to load the shocks or suspension
3. do you want it to help with any law of physics you can apply?
I wouldn't be too concerned with where it was as long as it complimented the setup of the car.
Why don't you fix several places for it and move it around to see what the car likes?
Too big a wheelstand, hang it in the front motor plate or front frame horns
Too much tire hit, hang it by the transmission crossmember
ECT.