Will Z51 wheel that are 19" and 20" it on a car with out Z51 suspension. And if so will it mess with any ratio that will affect the cars performance.
As Mark and Glen said, it will work okay.
To expound a little on Mark's answer, what matters is the outer diameter of the tires, not the diameter of the rims. So, with larger rims, if you put tires on with shorter sidewall heights, then the outer diameter of the wheel (rim+tire) will remain the same.
Are you familiar with the Pxxx/yyRzz tire measurements? It is a fun little exercise to use the xxx (width of tire in mm), the yy (sidewall height as a percentage of width), and zz (inner diameter of tire = diameter of rim given in inches) to show that, for both the Z51 and base StingRays, the rims+tires are almost exactly the same outer diameter (they are oh, SO close!).