I put one of these Mobile home furnaces in my garage a few years ago. I made the bottom frame out of 2x4's, figuring this is what they used in the mobile home, and had a duct box made at the local furnace shop, I have 3 ducts from the box, 1 to my office, 1 to my pump room, and 1 to the garage part, which I can shut off any of them if need be.
The furnace works great. The only problem I had, was when the Propane company came out to hook it up, they wouldn't, they said because it wasn't done by a licensed contractor, and it was a modified setup. No way was I gonna pay someone a grand to hook up something that cost me $40 to hook up, so I tapped into the line myself. Not wise for insurance purposes, but I only use it when I'm in the garage, and my Dad was a furnace man his whole life(retired) and he said the hookup was fine. This is just something you may want to check into.