Cylinder block hole sizes

The 5/16" cylinder head bolts (near spark plugs) on my 72 Interstate I'm restoring have only got 3/8" holes in the cylinder block. Has anybody come across this problem before. I'm guessing it's the wrong block but everything else fits fine.

Regards Chris.

Do you mean only 5/16" holes in the cylinder block which would indicate it was an Atlas block as 3/8" is larger than 5/16"?