I have a 67 Coupe with leaf springs that sag a bit. Currently there are air shocks on the back that leak. Looking to switch to a coil over shock and leave everything else as is. Not looking to replace the entire rear suspension. Any suggestions on what product I can use. I want to have the rear at a certain height to fit 255/60/15 so that the top of the tire sits inside the wheel well half inch. All the coil overs I search require all new suspension kit. Is it possible to just put a coil over in place of existing air shocks? Thanks.