Hi Don:
Everyone has there own thoughts and opinions regarding economic maturity vs. biological maturity. However, 47-inch DBH northern red oaks here in the Midwest would be considered "way overmature" for that species from a harvesting and utilization standpoint.
I would suspect that these trees have some significant butt rot, but they might be sound -- don't know, as I haven't seen the trees.
And yes, some trees are more valuable for firewood -- your call, because they're your trees!
Best, Jay