I just bought the OBX shorty headers off ebay and I am extremely pleased with the quality of them. The reasons I bought them were not for performance, but I needed something to replace my leaking cracked exhaust manifolds and didn't want to put out any extra money to modify the Y pipe.
There was an issue with the header flange, where they forgot to clearance around the drivers side front sparkplug, and the passenger side rear sparkplug. I have emailed them for comment, but have not heard back.
I modified them to fit my truck, but not too happy about having to remove them, cut them, and reinstall. Had to do the job twice.
Edit: The ones I bought were the ones for a 2001 Denali 6.0L, I don't know about the other engines.