Can i use stock mustang pushrods with my new gt40p heads?

They should work. I say should cause there is always the chance that due to some differing production tolerances or what something that was done to the heads previously, ie milling them down, that the stockers won't work.

I was able to reuse the stockers on my set up. You can always bolt them up and see what ya got.
 
It may depend on what rockers you use, ALWAYS measure just to be sure, ive heard many people using aftermarket rockers that had to get new push rods. Every car is different, measure just to be sure.