Ya, mine with a 350, aftermarket heads, cam, Holley 1850 with stock 66 jetting with the 700r4 gets 12-13 average.
I have tried different carbs (tuned Qjet) and cannot get much better.
I do drive 65-75 on the freeways and have some fun around town once and a while but I have come to the conclusion that these trucks with the old school carb'ed engines are going to get bad gas mileage no matter what. There are exceptions but very few and I want to know thier secrets
Heck, even my Chevelle with a hot solid cammed 427 will get the same on the freeway. Maybe even better, but not by much....