About five millimeters. I did this by severing the vertical brackets of the rear axle, and instead gluing in a pair of L-brackets made of heavy duty brass rod, to which I also rigged a small "shelf and a sideways brace of the same brass, and finally, I glued the original axle to THAT. I gave up on any representation of the rear springs, but instead went for a very, very sturdy arrangement.
I too would have preferred the arrangement you describe, sawing off the integral horse bases has been horrendous, and the legs are so spindly I don't know if they even can be pinned to the base, or whether I should just replace the equines outright.