I did a fair amount of research because at the time these were ordered it wasn't 100% they would fit over the ZL1's front rotors. I found someone on a Pontiac forum that put these wheels on his G8 with CTS-V frt. rotors, so I knew the wheels would work.
However, most of my offset info came from searching posts by members SUX2BU and SGOS252382 in the wheel and tire section. Nineball fine tuned my #'s as he has done many 5th Gen aftermarket wheel sets.
Front is 18x9.5 27mm offset, which works out to 6.3" backspacing
Rear is 18x11 47mm offset, 7.9" backspacing.
Overall tire width affects things, the M&H's are a narrow 325.
A 43-45mm rear probably would have been enough to keep from the slight rub on e-brake cables.
Stock 20's are 10" front at 23mm, and 11" rear at 43mm.