It looks original. A couple of other things to look for are:
1) the boxed in front spring hanger mounts. I own 2 1970 340 dusters and they both have them. One of mine is a 4 speed and the other is a three speed manual.
2) There should be two muffler hanger brackets on the floor pan on front of the differential (because 340 cars came factory with dual exhaust). It is where the back of the muffler is supported. The driver's side one is spot welded on and is standard on all cars. The passenger side one is bolted on for dual exhaust cars only. Also on the passenger side under the rear seat is the plate that the exhaust hanger attached to , it has three bolts.
3) In 1970 340 dusters came standard with SPB disc brakes.
4) The rallye dash has the thumbwheel radio which is correct.
5) The real truth would be if you could get the VIN number off of the radiator support