Tow Bars 71 Duster 318

I am looking at this bar from Harbor Freight http://tinyurl.com/p4z5so2

It is good to 5000 lbs and 71 Duster with a 340 weighs in a 3100 lbs. Mine has a 318, even lighter.
I am thinking about drilling 2 horizontal holes between each of the vertical inner sets of bolts holding on the bumper. I will put in a plug until I need it. Then I can bolt on the brackets as I am waiting.
I am also trying to think of some way to reinforce the bracket that hold on the bumper.
Not as heavy at the ones I built but probably will work for a short tow and not too fast. I would strongly suggest adding a thick flat bar, maybe 2.5-3" wide, across the 2 main bars and bolted in with 2 bolts on each end and about 3/4 of the way back from the ball; you could bolt this on when attaching. The issue with these bars is the adjusting bolts at the front; they can slip and the car will move off to one side or the other; not good! The added bar will triangulate the main bars and keep them from moving.

It sounds like you are reinforcing the bymper mounts; good thing; I would not count on the bumper metal and bolts; the bolts are usually carriage types and may not be a graded bolt. Make something that reaches back into the subframe.