I'm not familiar with specific roads and he ended in Abilene which is going to be in the pan handle and quite a bit colder. However, reading through report above and looking at the maps, there are probably a few ideas to borrow from. I'll also note my route is described in my journal: http://www.mvermeulen.com/oneyear/usa.htm might also have some things to choose from.

Routing and route choices is going to be a personal thing and preferences are going to differ. However, the route above used some choices that I would also do if I were time-constrained such as Ryan in:

1) Several portions of route above use US90 and US190. Those larger US highways are sometimes hit and miss. They are likely to be more direct than what Adventure Cycling prefers. Occasionally they'll be busier and every once in a while, they'll be really the wrong choice. So the trick is to blend things a bit and to ask folks along the way. When you get close, buy a state map and look at combination of the state map & adventure cycling maps. Make your choice, read what was in other trip journals and ask folks (ideally cyclists). If it really turns out bad, anticipate choosing smaller roads to get you back closer to AC routes. Often these highways such as US90 that parallel an interstate can be a good choice if major traffic stays on the interstate though you do have to watch out about road construction that occasionally redirects interstate traffic onto the alternatives. Also roads I've been on in LA, MS and AL frequently don't have much shoulder and some drivers not anticipating cyclists so one will need to be extra careful.

2) Both his route and mine had one or two awkward spots to get past: Houston and Mobile in particular. I avoided Houston in following Adventure Cycling route and he got a bit closer. Mobile on the other hand, I went south and around and he had some fun with tunnels around Mobile Bay. The Adventure Cycling route misses the entire issue by going a moderate amount further north.

So I'd still suggest he default to following a rough routing of Adventure Cycling southern tier, but occasionally make alternate and more direct choices looking at state maps. My own personal preference coming up:
= Mostly follow the adventure cycling route to Del Rio, though take US 90 from Van Horn as suggested before.
= After that, in Texas I'm less certain of choices.
= Gulf Coast, seek a compromise between Adventure Cycling and the most direct routes with special care
around Mobile and Houston.
= By Florida, US90 tends to be a good choice. Don't bother swinging too far south to St Augustine but instead
start looking to going up the Atlantic Coast. There might be wiser choices cutting across Georgia but I don't
know much there.
= Ask folks as you go along, particularly cyclists and get local info such as road construction.
The result is likely to be a somewhat more direct route that saves ~350 miles from what was posted on his web site without going completely further north and taking some more weather risks. With short hours of daylight, some spots using more direct choices such as US90/US190 will need to be extra careful and avoid riding after dark if you can.