What year is your block? From what I've found XB code 400s only went into 67 full size cars, XB 350s only went into 69 Firebirds. But if it is a service replacement that could be different. Ths service replacement ID is not in the stamping on the front but in the casting on the back. They didn't normally have an engine number stamped onto the front unless that was done when it was put into a car as a replacement engine.
