Welcome CannonKrazy,
I to found this forum by accident. There is a wealth of information here. My experiences with casting bronze have been mixed. The main trouble I've had is getting it hot enough. My first furnace was made out of firebricks, like you proposed. It worked great, for aluminum. But I had trouble getting the heat required to get bronze to a good poring temp. I could melt it, but the metal would freeze before the sand was filled. I think the firebriks loose to much heat?
I built a new furnace out of castible refractory. The walls and lid were pored 4" thick. With my old burner it would take 45 min to get everything hot enough to pore 660 bronze. I gust built a new, bigger, burner and just need time to test it out.
Hope this helps, Wes