Thinking long burn times with a boiler is rather backwards thinking. The only way to get longer burn times with a boiler is to cut the air to the fire - which leads to big time smoldering & creosote production, with that water jacket there cooling the fire off. The key is conditions that will allow a continuos hot burn, and efficient heat transfer area downstream of the burn.
The Benjamin has next to no heat transfer area. Check it out. It is just a cylinder that you load wood into one end of, and the fire exits the other end right into the chimney. I was a slave to one for 17 years and it would be one of the last units I would recommend to anyone - unless you have next to no heat load. I had to continuously tend to it, feed it, clean out coals (lots of hot ones), and clean the chimney, regularly.
Now I only have a fire burning 6 hours a day, and don't clean my chimney anymore. If you can't do a gassifier, at least get something with good heat transfer ability (usually involving tubes - but the Biasi would be worlds better than the Benjamin), and see if you can incorporate storage. If you can add storage, your world will change even without a gasser.
