What about 3D printing them? They have printers that cant print in a lot of different materials now.
This is what I teach. The technology is improving with carbon fiber reinforced plastics, High temp resins, and a myriad of materials.
At the end of this path you need to figure out the
1. Material properties you need (Temp resistance, strength for given purpose, resistance to solvents, etc.)
2. Size of the print bed
3. Surface finish
4. Printer capable of water / pressure / vacuum products. (Many look good but fill them up with water and they leak)
An alternate method that is easily used is to use a 3D printer to print the bucks to shape the sand in the molds for casting aluminum. This is what I am about to start on if I can find a donor Hemi head to get some measurements from. This may not be the method the OP posted about, but by using the two piece manufacturing techniques of plastic manifolds (notice most have a seam where they are glued together) and the readily available, easy to use aluminum material his end goal could be achieved and possibly be better in the end for small productions.
Can it be done in a 3D printer? Yes because they can print steel strong enough to make a Colt 1911 that works just like traditionally manufactured one. NASA even prints rocket nozzles. Rigid to Flexible materials and lots of temperature ranges are easily selected also.
Like everything it is just a matter of time and money.
