The numbers before the CRN would be the date of the block assembly 4 digits, 2 for month and 2 for day which would follow a leading "T" for Tonawanda factory ie. T0313CRN. You can check other engine components for dates if they are GM factory items - like the the intake manifold, distributor and heads if you pull the valve covers. If the stamped dates are before your VIN date month and week, chances are it might be correct.
The block itself should be stamped on the top back near the bell housing with 3963512 for 1970 454 and also have a date code stamped nearby for when it was cast that can then be checked against the body tag month and week. "Chevrolet by the Numbers 1970 - 75" book by Alan Colvin is excellent reference for correct part numbers and date codes. Jim