I have the ARH 1 7/8 headers (short system, just headers with catless extensions).
I THINK the only other company that makes Z/28 specific headers are Kooks (note that Kooks sells their header extensions seperately and I think you have to use their extensions if connecting to factory H pipe).
There is some rumors that the regular Z06 headers fit (and are cheaper even for the same brand) but ARH says that they have the same start/stop points, there is supposedly significant clearancing to avoid the oil cooler, other stuff that is different between the Z06 and Z/28. I could not find any evidence of someone actually USING the Z06 headers.
I think general understanding is that the 1 7/8 will work great for stock engine but that the second you port heads, go big cam and/or port/swap intake manifolds then the 2 inches are probably wise.
There are some good pictures linked in this thread of the entire exhaust as it is under the car.
https://www.camaro5.com/forums/showthread.php?t=572740