Welcome, that looks like a nice Duster!
Well someone put a 340 in some time after it left the factory since 1973 was the last year of the 340 engine. Have you actually checked the side of the block near the starter motor to confirm it’s actually a 340 block? Lots of folks sell a 318 or 360 V8 passing it off as a 340. Post the VIN of the car as that will tell us if the came with a 225 or 318 from the factory.