I must profess to never having built a band organ but the details of
organ pipe design cannot be too different from a regular pipe organ
so here is my humble response to the question:

When building a reed that is cut exactly to the resonant length it is
said to be cut "dead length."  But this is not as simple as it sounds.
First off, at what temperature was this done?  A change in temperature
is a change in tuning.

Secondly, the air column of the pipe is the length of the resonator
_and_ is also partly a function of the air column length of the
shallot.  Thus you will see a slide on the trumpet bells in a Wurlitzer
theatre organ Brass Trumpet rank.  This is the fudge factor in tuning
the resonator to the reed.

A bright stop on a Wurlitzer theatre organ like the English Horn
(really a French Trumpet) is cut dead length with no tuning slide but
since it has a French shallot there is a trick one can do to tune the
resonator to pitch.

A loudspeaker will not behave the same as a reed tongue.  It is a
driven device that will maintain pitch no matter what.  A reed tongue
will interact with the resonator and a phase relationship occurs.  You
would likely get differing results by using the loudspeaker to tune the
resonator to dead length and those results might be puzzling at best;
e.g., a pipe that "flies off" and won't come to pitch.

My suggestion is to use the shallot and reed you plan on using since
the air in the shallot will effect the total resonator length.  I would
make the resonator on the long side (tuned flat) so a notch can be cut
into it for a tuning slide that will allow it to be tuned sharp.  Then
you can be sure the reed and resonator are both tuned in agreement for
the loudest and most stabile tone.

By having the ability to tune the resonator and the reed you will find
when moving your organ from one temperature extreme to another you will
still have good pitch control and can compensate the temperature change
more easily with just the tuning slide on the resonator.

Kindest Regards to All,
Al Sefl