All depends on whether it is a 428PI 8V (used in 1967 SHELBY MUST) or a 428 COBRA JET 4V (used in 1968).
The only production dual quad 428 was installed in the 1967 GT-500. The 68 saw the CJ (single 4V) (early 1968 production was the 428PI from the previous year I believe).
He needs to offer more info.
Let me correct this. The early 1968 GT500 had the previous year 428PI 8V installed. The 428CJ engine was not introduced until the spring of that model year, so the engine advertised should be the 355HP version. The CJ was rated @ 335HP (they cheated a little bit on the true rating - more attune to 400HP).
If all is true, it is well worth the money and yes it will fly as I had a 67...