Fireplace installation

I have read this group and others on installing fireplaces but I still have some questions. I have a victorian cast iron insert and wooden surround to reinstate in a victorian house. The insert has a 'bolt-on' cast iron fireback. My main concern is how do I fill behind the fireback and make a nice sloping throat etc. If I detatch the fireback and use it to shutter the infill etc. then will it not become affixed to the concrete? I am also unsure exactly how to fix and seal the insert to the wall. Any one know of any good websites with detailed instructions? Or have similar experience? Cheers.