Steve, when I bought my boat 5 years ago, it had a dead Lectra San that I replaced with a holding tank. I only have one head, where I think you have two. Shouldn't matter too much. I kept the same toilet.
I did the whole job myself. Not really that hard. Installed a custom 30 gallon tank under the queen bed forward, ran the lines through the PVC cutouts already in the bulkheads in the bilge. Installed a deck pumpout through the anchor locker. And a mascerator pump to pumpout at sea, when I am offshore. For me, the key was getting the tank from:
http://www.oceanlinkinc.com/ They install the tank fittings exactly where you want them on the tank. Good quality tank.