Tongue and Groove Beadboard Porch Ceiling Direction?

[purplehouse]

I am removing our 100+ year old tongue and groove fir beadboard porch ceiling due to significant cupping (porch settling plus fill boards between ceiling joists failed) and water damage. We have been going back and forth between replacing it with fir but are leaning towards a grade D pine 1x4x16 (or 1x6x16) since it will be painted.

Originally the beadboard ran perpendicular to the ceiling joists in lengths of 5' 6" (3" wide with center bead.) My qustion is if the porch continues to settle should we continue to run it perpendicular or run it parallel? If we run it perpendicular what lengths should we use to avoid as many joints as we had? Porch is 25' long and 5'6" wide.