I have a 68 Coronet R/T that was built on July 15 at St Louis. From what Ive been told by the "experts," my car was built on the last day of production at STL in 1968 so you also have a pretty late build car. You could have some 1969 parts on it like the leading edge of the hood should be drilled for the '69 grille insert and your door buttons might be chrome instead of black.

I think the 90 day rule is pretty accurate, my block has a May 1968 casting date but the VIN is not stamped on the oil pan rail and I havent checked the back of the block