any advise
Ferrea 6000 is their best valve. But it is not made with American stainless steel material. I think it is Italian or Israili made.
As soon as you think Nitrous you should think MANLEY. The cycle life of the extreme duty Manley exhaust valve is many times more than any import valve because of the quality of the material.
Yes the 6000 is $10 a valve and the Manley is $20 a valve but the engine is many $$$$. Go save $160 somewhere else than on your valves.