If you can secure a set of the 920 casting heads from a 273, they're about the same specs, never heard of them cracking and nobody wants them (everybody wants 302 heads). Might look in to it.
Sounds like a good backup plan. This is my first time dealing with cmegines, the guy seems nice but there is a lack of communication. He never called to update me that the cores that they found were cracked and it's going to take more time or that they are having issues finding other heads, I had to call them and ask how the heads were coming along.