Interested in replacing the wheels on my '99 c5 with the newer 2009 c6 wheel. I have read that spacers are needed to mount the rear wheels. Anyone know what size spacers are required? How about the air pressure sensors - Will they need to be replaced with the ones from my 1999 c5?

Welcome to the forum ssrodder!
I believe you will need spacers but can't recall the size. Try the Search feature above.

Yes spacers will be needed. The wheels will stick out beyong the wheel wells and throw debris on your body work.Additionally you may need to tub your rear wheel wells to accomodate the size of the tires. Go with replicas if you gotta have the style in C5 sizes
Yes you will need to put your tps in the wheels. There are kits to upgrade your 99 sensors and fobs to the later type for less than the price of the sensors alone. Think twice about the wheels.. best of luck