It appears to be a 'Thin' case for a 1963 Impala. If you had the 'Thick" case variety it would have a 63 cast where your single 3 is now. Thick or thin has no bearing upon the strength of the case but was determined by the foundry the poured it (both cast the housings to the same GM specs). The number of screws on your posi housing is the important feature as it tells you if you have right or left hand threads holding the case together. Yours should be the new and improved 10 plate clutch with 10 LH screws holding it together. IF it has only 8 RH screws then your posi has only 8 plates. If your carrier has casting number C-30743 it will hold a 3.08 to a 3.70:1 rear gear. Casting number C-30751 and it holds a 4.11 or a 4.56 (that was as high as the factory offered).