I do not know of any free site survey tools for predictive surveys. If you have a few APs to use, you can do a physical survey with some manual work. It would involve starting in one corner and placing APs. Then you see how far coverage reaches with your required RSSI. If you have a cisco card they have a site survey tool to help with this. Then you move the APs to the next possible spot and make sure you have enough overlap in your cells.
If you decide that this is a bit more than you bargained for, I highly suggest looking at prices for trained site surveyers, because it will cost more putting APs in the wrong place and re-doing it than a good site survey should cost.
If you want to look at predictive tools, I suggect looking at Ekahau and Airmagnet. They each have products to do predictive.
As for the firewall, I know cinder blocks that thick will drastically impact signal, but I have not seen them fully block it. Buildings with that as the internal construction type tend to require many more APs than stick construction.
