I would guess the early heads for a 1968 would be worth a little more, due to being more rare. And Ford made CJ heads for quite a few years after car production ended, those dates probably a little less valuable since they wouldn't be correct for a restoration. However, I would say condition will be important too in figuring value. A set needing lots of work, or that have been milled excessively, or the subject of a bad porting job, will all be worth less regardless of the date codes, especially since CJ heads aren't extremely valuable, so quality of the work will matter too.