If you have real bucks to do this, why not machine it out of billet aluminum. If you are casting your own engine with no experiance, this would be the safer and no doubt cheaper route, remember you'll have to come back and machine every important passage and surface anyway. I bet you could cad model it in such a way to give access to all the prots and such, bolt on, press on, everything else, you may need a couple covers to gain access in certain areas, but for a one-off production, you'll spend that much just machining the molds to make the blanks and then you'll have to worry about bubbles, cracks, incomplete fill, voids, and why go through all that.
Industrious project though, good luck.
I just wonder what you are thinking you can do better than 100 years of engineering and manufacturing design? I mean GEN I sbc has been track tested for 50 years... you could generally get any size and configuration you could desire with as much money as you will spend on fabricating one from scratch, although for a one off show car it would be a bit more prestige.
Sorry for the lecture, I just don't understand the reasoning behind this endevor.