The thing is, a cast material change may well be whats required to fix the issue, but shrinkage rates post casting on another material may nett some other issues as yet unknown. It has porosity issues as is on the existing material, so gating and or the whole process used may be suspect. I have been thru casting sagas's an iterations in my day job previously, and its not uncommon teething issues, BUT, the material chosen looks to be as much use as a chocolate teapot. 30-40amps on my tig and this stuff was falling away like melted cheese, and outgassing/fizzing as the contaminants came to the surface. the Oe manifold I tried as a comparison had no such phenomena and needed higher amps to melt/weld. Crude test, but confirms my belief.
there are materials like what I googled below which are designed for the high temp dutys they will see.
http://www.cbmm.com.br/portug/source...s/pdfs/053.pdf
Other comment on the replacement shows a build up of sand on the mould from the previous cast on the webs, which to me shows there are cleaning issues of the core at the foundry if this is not checked. All symptums at face value of a very cheap foundry process being employed.