can martha's mac and cheese be made a day ahead, and then baked or will pasta absorb all the béchamel and be dry?

My experience has been that letting it sit and then baking the next day- is fine. I might be concerned if the pasta were sitting in cream or milk or stock, but beschamel is thick and contains flour, so the pasta won't get mushy.

Better to bake and re-heat then assemble and let sit overnight