TxDot has a wonderful history resource for U.S. 80 going back to 1927. While it does not map individual small alignment changes it does give routing through towns and cities along with the mileage between the cities. Alignment changes are indicated from the differences in mileage between towns over the years. In some cases the exact street names through cities are specifically noted. Enjoy!
http://www.dot.state.tx.us/tpp/hwy/us/us0080.htm (http://www.dot.state.tx.us/tpp/hwy/us/us0080.htm)
That TxDOT site is cool. I've used it for researching the old farm-to-market road sections of the OST. Very helpful. Thanks.