What type of rear suspension does it have? You said it currently uses a stock non-adjustable rear spring which on a Nova would've been leaf springs. It sounds like an odd setup. Most backhalf cars I've worked on use a coil over shock to start with. If it is a leaf and you're going to coil over do you know how far behind the rear axle the coil overs will be? Also if you're removing a leaf spring there's a good chance that it doesn't have a track locator on it because a leaf spring will keep the rear in place. If this is the case you will have to add a track locator of some type.