Hands down the biggest bang-for-buck mod I've done to my 67 Coronet wagon is a Hellwig rear anti-roll bar. On the first hole my car stays flatter than my wife's Subaru at normal to safely-quick city speeds, and even let me hang with friends driving moderately in G35s and Mazdaspeeds (on 215/75s no less). Hardly any change at all to the ride, even with crappy Monroe blues. Regardless of which bar you choose, these big booty cars really benefit from them.