what year was my engine built

number on back side of engine: 3999289, number on id pad on front of engine: T0604TRK I looked up the number on the back side of the engine on an internet site and it said 72- 78 so i looked up the id number on the same site and found out it was born on june 6 in the tonawanda factory (T0604) and the TRK had 3 different years listed for that suffix:73, 78 and 81. i figured its either a 73 or a 78 since it fits the years for the casting number on the back of the engine. any other places going to tell me what year it is?