Concerning your question about the overfire tube.
The overfire tube comes from the airbox, the air comes from the combustion fan that blows air through airbox and then through the tuyeres. What percentage of air comes through the overfire tube would have several factors. How clean is your airbox (is it full of ash and rocks). Also the air holes in your tuyeres, have you cleaned them to get the small rocks out of the holes. Sometimes they can get so full of small rocks hardly any air can get through. If everything is clean you would about 10% of the combustion air coming through the overfire tube. It blows air down on the fire and spreads out the flames so that it heats the outsides of the firebox better thereby making the stoker use the heat from your fire more effentiently.
You could add an overfire tube to a combustionaire or stokermatic but you would need to break out the refactory cement in the bottom of your firebox, then drill a hole (the hole goes against the leftside near the back) for the tube in fit down into the airbox and secure it so it doesn't fall to the bottom of the airbox. Then put new refactory cement on the bottom of the firebox.
Hope this is helpful.