If it says 3.5 on it I would bet it is a 3.5 unless someone has sleeved it down. There is a date casting number on the exhaust side of the block that can help you track down the original year of the engine, if I recall correctly it is something like 10A82 would mean Jan. 10, 1982. Keep in mind they apparently never heard of just in time in those days and castings were typically 6-10 months before vehicle production dates. Hope this helps.