i did this same conversion because i was junking a 74 duster that had the fold down seat so i moved everything over to my 73 duster, and the only problem i came across, is that its not as simple as swapping the package trays out because the package trays have the last six digits of you vin number stamped on them so it is a good idea to keep the numbers intact so i ended up cutting the original tray out at the speaker holes and leaving 2 inches of metal in front of where the number is stamped and welded the curved section from the donor car. had it not been for the numbers i would have just swapped out the whole tray and saved about a week of work. Other than that if you have a donor car it is really easy, just time consuming with a lot tack welding and fitting everything right then going back and finish welding.
