Not sure I would mess with it unless you have a manometer, and a way to measure flow when you go to cranking on that gas valve. It is calibrated for NG now, unless you already tweaked it.
Then in the manual it says;
"This conversion should be performed by a
qualified service agency in accordance with the
manufacturer’s instructions and all applicable
codes and requirements of the authority
having jurisdiction. If the information in these
instructions is not followed exactly, a fire, an
explosion or production of carbon monoxide may
result causing property damage, personal injury,
or loss of life. The qualified service agency is
responsible for proper conversion of these boilers.
The conversion is not proper and complete until
the operation of the converted boiler is checked
as specified in this manual."
The NG gas company may also frown on you doing it.
The regulator that Reach4 posted should work, but the 100 # tank would be best.
You will be lucky to get 4 hours out of the 40 # tank.
Have Fun. Be safe.