That would be March of 68. The block would have a 8C and then the day after that cast under where the
oil filter remote bolts on to the block. It could be like 8C5 or 8C25. If you could find a guy with a April or later built 427 Cougar it might work for him but there would be very few built in that time period.
Let me know if we can help with somehting.
Thanks, Keith