I-20 does not either, stopping (or beginning, depending upon how one looks at it) near Van Horn (or more precisely, Kent, Texas).
I-30 runs only from Texarkana to West of Ft. Worth. From there on (Westbound) the road designation is 1-20...
Distance of Interstate 20 across Texas = 636.1 miles
Distance of Interstate 10 across Texas = 878.6 miles.
I understand your correction. And that is the way it is today. But, in 1967 (and aroun 1970), the milepost protocol must've been different.
There was a milepost 838 at Texarkana.
878 miles to El Paso.
985 miles to Chicago.