My understanding is that starting with 1969 Federal law required a windshield defroster. Thus the heater delete option was eliminated. This would also explain why that sheet is NOT in the assembly manual. However, they did still produce the dash and firewall pieces as a service item.
Yes, the firewall pad would still have that area covered.
While I'm certain that some knowledgeable racers certainly did order 68 and earlier cars with the delete option, most probably did not.
However, my experience is that most heater delete plates have come out of regular cars in the state of Hawaii.
The plug used to eliminate the heater hoses from the engine had the 3/8 square hole, the same as used on the passenger cylinder Head of a 68 up small block. These tend to be hard to find new as most replacemends are the hexagon version. When I did mine some 20+ years ago I found the correct new plugs in an electric motor sales/repare shop. New electric motors have these plugs screwed in where the wiring conduit is run. The only difference is these are galvanized coated as I doubt the originals ever were.