I think I remember seeing some flat top pistions on the Stillen web site. I know Nismo makes some, but you will pay dearly for those. If you do find some, let me know. I'm thinking about doing the same thing to my '83 n/a. However, it's been a long time since I've dove in to a motor that far and am a little hesitant.

 

The 1981-83 n/a 280zx engines already have flat top pistons. You can use some stock Nissan pistons, or if you have to bore out the block for a rebuild, the KA24DE pistons from about 1992+ are flat top and a 3mm overbore for the L28. They are from a 4-cyl engine so you'll need six pistons and two ring sets of four.

The KA pistons might be cheaper than getting overbore 280zx pistons.