Didn't GM offer a Lock up option in a TH350? Was thinking that they did in the mid 80's kinda odd... Seems I looked at a truck back 10-15 years ago that had a 6cyl / 350TH with a lock up and the guy said he got mid 20's just running around.
My 79 C-10 with a 5.3/60E/3.73 gear gets 20 on the highway. And I drive it like I am late all the time
At least getting up to 70ish..