They look like those Chinese headers. A friend of mine tried them on his 87' GN and the fitment was pretty poor. The turbo wasn't in the right place and they didn't clear his 3" down pipe. There were some other issues that I can't recall. I would stay with the stockers unless yours are in poor shape.
They are a knock off of the the old ATR (Applied Technologies & Reaserch) and Kenne Bell headers.
They do take a special downpipe, I believe RJC Racing and GBody parts will fit.
For that price ($<200) you get poor fitment, inferior materials and weld quality.
If you MUST have this style, check out GN1 Performance.com as they sell them.
The 15-20HP you gain is only if you put this style header on a 9 sec ride. Stock headers are good to low 10's.
Search Chinese headers for more info.
I have a pair. Someone modified the cross over pipe. If the v band would seal on the passenger side, I think they would work ok. There's other "name brand" headers going to $600 that are made in China anyways.