Bob the answer to your question is that this needs to be permitted from your local building dept. While there is a code in the state which I believe is 18" front and sides, I believe the rear is more but not sure. I would call your local building inspector or even fire department and ask them what they'd LIKE to see. It wouldn't hurt to make it more.
I do know that you can vent it directly sideways as long as the outlet is 12" out and above grade...it can go anywhere on the side of a house but you may be required to maintain a certain distance from a window or a door-this I don't know.
You have to keep a minimum distance of non combustible materials around it. Stone/Mat under it, Wall must be padded or covered with metal behind it etc. The long and short is I bet you could fit one in quite a few places if the material around it was addressed properly which you would need anyways for the inspection.
This is a link for the State stuff:
http://www.mass.gov/?pageID=eopsterm...t08&csid=Eeops