We screwed them on using 1-2 two inch wood to metal screws per board. The problem we ran into was, although we carefully selected all of our wood, some were still slightly warped. We started in the center of the ceiling, because I wanted a straight line down the center. However, when we reached the sides, over the window, there was a significant slant. We will cover this imperfection with trim, but if I were to do it over again I would start at one side and work my way up and over. You will need two people to hold the wood in order to keep it straight, and I would recommend keeping the screws loose until you have installed all of the panels... This is because the curve of the ceiling makes it hard to insert the tongue to the groove when the panel is tightly in. We also put a layer of insullation tape on the ribs so that when the bus is moving, the wood and metal won't rub and make noise- I definitely recommend this!
There have been no issues with expansion and contraction thus far, but we need to seal it soon to keep the blue stain fungi from spreading.
I love that dark stain too, it looks really classy! We are just going to have a dark floor, so we wanted a light ceiling.
Have you removed all of your ceiling panels yet?