You will not get gyno if you take enough arimidex. It is about impossible. And if you take 40mgs of novs and get gyno it would be highly unusual although possile.
Arimidex doesnt block estrogen, it stops test from aromatizing into estrogen. Complete aromatization is not a good thing. On 500mg I would take .5mg ED to be safe. Have nolvadex on hand as well, but I doubt you will need it.