Look on the left, driver's side, of the block in the starter area. There will be raised numbers cast on the side. The engine size will be there, ie, 318, 360, etc.
Whatever you are reading is wrong. Most '74 Dusters did not come with 360's. The exact opposite actually. A '74 Duster 360 would be a pretty rare car. Around 4,000 were built out of 280,000 total Duster production in 1974.
willing to bet its a 318 if its factory motor.
Yes..its cast in raised numerals on the side of the block but you might have to get under the car to see it.