I have a competition engineering rear coil-over suspension on my 68 Nova. My question is if I take the leaf springs off and put on ladder bars will the rear-end be stable? I feel that I'm missing something. This is the first real major rear-end modification that I've ever done. Will the geometry be correct? Also how about the cal-trac bars?

I would ASSume you need a lateral locator(PHR, watts link etc)

You'll need some sort of diagonal link/locator/track bar/panhard bar. Something needs to keep the diff from moving side to side. The most common installation is the diagonal link. It's located from the front mount on one ladder bar to the rear mount on the other. This triangulates the suspension and prevents it from moving side to side.

You can keep the leaf springs and still add ladder bars. You'll need a floater kit to do it.