Thought I replied in here already (must have done so when Rich was cleaning house or something and the lag got me).
Anyways... clearly the manual calls for 18" to any combustible period (brick, metal, rock board, etc. is not combustible)... I do not wish to start saying things that counter what the manual says regarding installation because I get to many emails from insurance companies and i always simply go by the book "as written then" as they are accepting of that. Its easier for someone else (whos name is not Crane) to speak freely regarding their personal opinions (as I do about other things).
The 44 can run very hot (top temps to 800 NP)!....your not going to want any combustibles around this thing if you think your running it as hard as i did (I used to turn my face loading the lil'sucker to keep my face from frying
) it can also run fairly cool (top temps down to about 150). I clearly should have had a larger stove...
I know none of this really does anything for you other than giving my personal sample top temps and saying read the manual. But honestly using common sense goes a long way and seeing what others have done in similar circumstances is wonderful to know.