Kevin, in a 5.1 setup the side surround speakers should be a little farther(maybe about 2')back than even with the listener, to lend a little back effect in the absence of 7.1 back speakers. Since you're going to be sitting almost up against the back wall, the best position is likely to be on the back wall about 6' up and separated about 10'. If, as you indicated, you might use a ceiling mount, position them similarly.
Since you have the 14' width available, it'd be a good idea to separate the front speakers a little more, about the same 9-10' that you'll be away from them.
