Treat it just like a 'small' radiator.. Take it to a radiator shop, have it pressure checked and cleaned.. . repair if necessary. When I did mine, it was found to be good (no repair needed), so they cleaned it and returned it. 68C is March of '68, and is perfect for your March car. My 09C (Sept '69 car had a '69J' in it... same month as production, so that may have been the 'norm'??