I did the method of measuring down the spark plug hole several times and each time i got 3.75. Upon further inspection, it does appear that the last digit is a G for the heads. The engine cast number might be C4AE-601B-A instead of C4AE-6015-A. The casting is just too rough to distinguish. Is there any method of checking the bore w/out pulling the heads or the oil pan? It doesn't appear that this engine was ever apart, but it is 40 years old.