[viking427]

.."The 700R4 with a 3.31 gear and a 28" tire "..

I would agree with this also.

Ive seen the prices for 12v 6BT's slowly climbing over the past 5-6 years. Not too long ago you could find them consistently in the $1k-$1500 range, now theyre typically $2k+. If your looking at a 6k lbs + truck and/or 4wd, I would choose the 6bt for a few reasons;
-More shops are familiar with them and how to diagnose/repair/tune them.
-Theyre still cheaper then 4BT's, overall.
-MPG will be dictated by engine rpm. With a 6bt, you can run a 3:08 or 3:23 etc. and the 6bt will have the low end grunt to pull it without lugging, if you tune it for lower rpm. Texas is a big state with 70-75mph limits all day, youre gonna want the ability to run those speeds, even if only a few days out of every a month.

If you end up with a lighter vehicle, then certainly the 4bt should be on your short list. You could also use compound turbos on it if you wanted best of both worlds, and you have the mechanical aptitude/confidence to tackle and tune it.