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No clue. The O/D ratio should be listed if you google spicer 6453.(edit, Spicer info here)
http://www.pgttruckparts.com/Spicer-TTC-6453-A-5-speed-transmission-s/2523.htm
Looks like .78
Also, for O.P. the newer 939 series trucks have an overdrive T-case. That could be an option too.