From Citroen SM Wiki

The original radiator is very high quality, and has very good heat-transfer characteristics. If at all possible do not discard it; have it gently & carefully cleaned rather than re-cored.

If it must be re-cored, be careful that the new core is not significantly thicker than the original, particularly if you have the later, larger, fan motors. They will be moved backwards and will interfere with the gearbox support/cross brace making the radiator impossible to fit.