What exactly is the manifold made out of? I may have found someone who can custom make a equal lenght manifold but he needs to know what it can be made from. I know cast iron and nickel are two but those are really really expensive.

Nickel is a plating like chrome. Custom made exhaust manifolds arent made from cast iron. Casting is only for high production parts. Like when you need 1,000 made. When you need 1 made you weld some steel tubular bends together.