Some of the v8 trucks had a more highway friendly 3.38 rearend gear. I don't remember where I got these codes but am told they are stamped on the outside of the rearend.

MA 3.90
MB 3.38
MC 3.90
MD 3.90
ME 3.38
MF 3.90
MG 3.90

These were listed for 1961 models. Hope this helps.