Cast from what? a fairly basic cast iron is most common but usually doesn't fit with 'tubing'. Cast tubular manifolds also exist in iron and other materials, if it's a high performance turbo car then the OEM manifold is sometimes a Ni-resist cast iron (like some turbine exhaust housings) and some OEM stuff is stainless but they virtually always use a ferritic grade like 409 which, like most of the less common flavours of 'stainless', is magnetic and will 'rust' lightly but not rot like mild steel doesIf the manifold is some flavour of cast iron then a nickel based filler is the best choice, that said steps can be a good thing... as long as they're in the right direction and not massive they're not gonna hurt gas flow* and can actually help if pulse tuning and the steps are around the right lengths from the exhaust valve
* along the same lines as mirror polishing after match porting being a cosmetic thing rather than actually producing useful gains in gas flow
