simply put will gears from a 96 z work in a 99 ss?

 

all the gears that are designed for the GM 7.5" rear end will fit in other similar rears.
the only difference is the size of the carrier.

there is a 2 and a 3 series. series 2 were cars equipped with 2.73s, and i think series 3s were the 3.23s and 3.42s.

basically you cannot (without some kind of spacer) put gears from a 2 series on a 3 series. and the 3 series will not fit a 2 series.

warning:
i could have that backwards

 

That's backwards, but you're absolutely right. 2-series gears (the ring gear) are thicker, so to put a 3-series gear on a 2-series requires the spacer.

All you need to know is what were the original gears in that '96 diff. As long as they were the same as what you've got, then you're good to go.