Looks like factory weights are similar for Dusters, last time I had my '70 Duster on a scale it was 3480 with me in it and about 60 lbs. of tools in the trunk and a half-tank of gas. So minus all that it would be around 3200-3300? My car has a B-body 8 3/4", big heavy 15" Rallyes and fat tires, factory undercoating, power steering, and a big 50-lb battery which will be relocated to the trunk. Engine is a 5.9 Mag with Eddy aluminum heads and intake but I haven't had it weighed since getting the current engine installed.
I agree find another scale to double-check that does seem pretty high. Although that factory A/C setup is around 100 lbs. with the big-*** compressor, evaporator and condenser, drier, and all the lines. A 727 is a decent bit heavier than a 904 as well. What rear end is in the car?
A trick you can also do is measure the front and rear separately to get an idea of weight distribution, just drive the front wheels on first, get a weight, then drive over till just your rear wheels are on the scale and measure again.