I live in Missouri where Great Plains Rat snakes are somewhat common. I have seen nearly as much variability in their coloration as you see in anery corns (no two really look alike!). So, I would hesitate to make any distinctions based on the two photos shown above. It has been my experience that Great Plains Rat snakes have lots more black checks on their ventral scales than do corns. However, in Texas, I understand that there is a population of GPRs that have no ventral markings at all! In short (too late!) I would feel better about making an I.D. via internet photos if we could at least see the ventral scales?
As to the last post about the corn looking snake found in the barn, I believe that most rat snakes (black, yellow, corns, GPRs, etc.) exhibit those similar markings in their young. A juvenile black rat looks very much like an anery corn at first glance. So, that snake in the barn could be just about any kind of rat snake (corn included), but there are subtil differences that identify each type, too numerous to go into here.
Hope this helps -- Darin