They are C8AE 6090-H drilled 14 bolts on the exhaust pattern and drilled for Thermactor. I see them for sale at $300 - $400 a pair. They were the same part used on 1968 390GT Mustang / Cougar / Comet / Fairlane so not only were lots made like that, lots of people still need them. If I had a 1968 GT500 I would be looking for a properly date coded pair. This would narrow the field significantly and it might make you consider paying a higher price for the right part.
The same castings were used on all sorts of low performance Ford, Ford truck, and Mercury products so a clever machinist can make the less valuable 8 bolt / non - Thermactor versions fit and work.
If you need more performance these heads respond well to larger valve sizes and porting.
Ok thanks. Nothing on the left coast goes for a 100, but I get they must be common.