Hi John, My separation for this conversion was this post. http://www.airforums.com/forums/f427...0sp-41537.html
Instead of a hack saw to take the air box off the the furnace i used a air hammer with a chisel to break the weld.
Photos: 1 old furnace; 2 new intake-exhaust done; 3 new furnace on old air box; 4 angle it's mounted on.
I set the old air box in place on the floor where I wanted it then set the new furnace on top. Using pieces of wood I shimmed the new furnace into the correct angle to fit the old holes(some adjusting needed) and the angle of the trailer. Once I had everything fitting correctly I used the parts of the old furnace for the gaps in the back (see link above) and bent some sheet metal for the sides. I took the wood out once I had the side pieces attached. I used pop rivets for this but sheet metal screws will work fine it's just what I had that was handy. Be careful where you drill into the new furnace. Keep in mind that this does not have to airtight it's just hot air. I did use Vulkem (Tempro) to fill any gaps that I saw. Hope this is somewhat clear. If you lived closer I'd come over and help. Good luck!