The engine number and the engine number tag pop riveted to the body of the car are supposed to match.
This is FORD's way of identifying engines for their Waranty program.
IF it were me and I saw unmatching numbers on the car it would SUGGEST to me that the engine block was replaced previous to the current owner or owner's knowledge with an engine block/engine out of another Pantera.
It may be that the engines were installed "hap-hazardly" in construction, but that seems less likely and wouldn't be my first thought.
Certainly if I was looking to purchase the car with a mis-match I'd think that the engine was replaced since installation in production regardless of what the seller was claiming.
By the same token, complete originality on these cars is not what I'd expect or even want so matching or unmatching engine numbers for me would not add or delete value in my eyes. It's ALMOST irrelevant.