From what I've seen, Texans and Easterns are fairly similar in size, dispositions and habits. As Jeff mentioned some Florida keepers of Texans have run into some confusion with Florida Fish and Game agents (Chris?) as far as being able to tell the difference between the two. If you know what you are looking at, it's a no-brainer, but apparently these guys don't always get to see a lot of Easterns. Easterns can have red (orange), black or occasionally white chins, and Texans are somewhat variable, but one thing that doesn't change is head scalation. The Eastern is easily identified by the fact that the 3rd supralabial scale is cut off from the ocular scales because the 2nd and 4th supralabials meet above it. If the 3rd supralabial scale (counting from the corner of the mouth to the front) contacts the ocular scales, it's not an Eastern. I've never seen one vary from that. Sometimes these guys want to get into counting dorsals, ventrals and subcaudals. I think they are just trying to show what they know, or hoping against hope for that "big bust". Anyone that has seen both can tell the difference at a glance.
I think probably the closest Central or South American Drymarchon to an Eastern would be rubidus, as far as coloration and temperament. They are difficult to tell morphologically from Texans, and scale counts may, in fact be in order. Rubidus normally has more white and the entire venter, or nearly so, can be white or cream color. I found that out recently when I purchased an adult pair of "Texans". I posted pictures on here and some of the guys said "Hey, those look like rubidus". So, I went and found (actually was sent) the scale counts for erebennus and rubidus, and sure enough, they are rubidus. I'm not disappointed, because rubidus actually appear harder to find and obtain than Texans. I had intended to find a pair and add them to my breeding projects anyway. Hopefully next year I'll have some rubidus babies to trade for Texans, etc. The pair I have are huge, ravenous and puppy-dog tame.