Check with your local Building Inspector for you own safety.
Are you going to make a room for this boiler? I would! And I would make sure there were no combustables anywhere near it.
I would make sure the you raise it off the floor.
Checking with your inspector will insure that it is done right and it is safe and you might get a few pointers about how to maximize your output.
You may have to get a permit for this work too.(small price to pay)
I would also check with the manufacturer of the furnance to see what they recomend.
I would also contact my insurance company and find out if this will raise your premium, (it might a little, or with the proper permits, maybe not). In the event of a fire, they may not cover the barn and any collateral damage caused by it!
I know this sounds extream but, take it from one who has seen it happen, it is more than worth it!