Have you thought of using one of your N42 heads?
Valves are heaps easier to get out of USA, both standard & oversize.
If your going to have flat top pistons your compression is going to be far to high to turbo, but with dished standard 280zx turbo pistons it would be good, just cant put over a bar of boost into it. If you build this engine right, you wont need anymore that 10psi max.
I'm building an L28ET motor. F54/P90 combination with just under 9:1 compression I'm expecting about 250RWHP, but over 500Nm of torque on under 10psi and with our cars weighing a miserly 1250 odd kg, that is going to be a handful in anybodies language.
Cheers, D