What you have is known as "integral" headed 250. The intake/exhaust is "integral" with the head. The earlier engines have separate intake and exhaust manifolds. They ARE notorious for cracking, and exhaust manifolds fall apart regularly.
The only REAL difference between them ("early" and "late") is the head and manifolding. The short block is pretty much the same. Using a head from any Chevy or Pontiac application from previous years will "work". The 194 head has smaller chambers, so watch static compression. 230 and 250 heads (as well as 292, HD truck) are much the same. The '73-'76 models have "hardened" exhaust seats. This is needed for the catalyst to work correctly (operating temperature must remain higher).
I would try to locate the correct head. It should be easier than any other approach.
Jim
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