Hey Ben,
you may want to do this a little more scientifically. Can you get the weight of your car's front and rear ends (with the new batteries loaded)? You could use this to compare to stock and select an "ideal" spring rate. Once you've got that, then you can start looking at the other options like the wagon or other cars.
You're pretty good at hacki...er... fabbing things up from scratch - have you thought about adding a second spring to each rear corner in parallel with the stock ones? Do you need front and rear to be stiffer?
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