It is most likely a 304 or 345.
Does not look any worse then mine when I started so see no reason to run away.
The 392 is unlikely as it was not at least a standard offering at this time in school buses. Also at this time it would have been before they sorted out the cooling issues. I had one and gave up on it after it melted two pistons, and put a 345 in instead. The trouble was hot spots on the cylinder walls because coolant flow between the very close cylinders was not sufficient. The 392 was the largest that started with a 266 in the same overall size block. The later improved cooling 392's were good engines.