By way of comparison, today I read 181.1 River Miles. Vicksburg at Mile Marker 437.1, Port Hudson 256.0

Difficult to believe that we could lose ~ 60 miles over the years

Historic Names and Places on the Lower Mississippi River
http://www.mvn.usace.army.mil/pao/history/MRnames/MRnamesTOC.htm

I'd try the Corps. of Engineers Archivist. I believe they have a station located between Vicksburg and Warrenton.