Hi Steveo
The Ebay manifolds are pretty poor quality according to people on the Nissan Silvia site who have purchased them. Apparently they need some grinding and general mucking around so they fit too.

Why do you want one? The standard manifold flows quite well. The owner of LOSTU has done some flow tests from memory. He is on the Performance Forums site occasionally. I recall reading a post from him where he said it was really only worth upgrading when most other things are done and you're chasing BIG horsepower. It may be worth having a chat to him before making a purchase.
Regards Matty