Could someone ID this engine for me?

S/N T5003CTX, or at least that's what I am told! Tanks in advance

T = Tonawanda engine plant
5003 = not a good number. Posibly 0503 for third day in May?
CTX = 2 different engines but you should be able to tell if it's one or the other.
CTX = 1975 400. LT4 175 hp. Came from A and B body cars. Has a 4 barrel carb.
CTX = 1978 305. LG3 145 hp. Came in G body cars. Has a 2 barrel carb.