The tire rack link didnt open for me however i did some research a year ago on this and found that there is quite a bit already done by the corvette guys on deciphering pre 1970 tire date codes. Yes there is a way to know 68 versus 78, 88 and that is by the number of characters in the number. The longer number first appeared in 70. So since you are collector extrodinaire of 68's then its a little easier