Well, I did it. I made a trip to monument valley and Grand Canyon last week. The goal of this trip was to see the remains of the arch. I am so glad I saw it as it's not much destroyed than what I saw on some SW web sites and on the OUATITW Bonus DVD. Thanks so much to Warren Oates on a previous post, he indicated the exact location very precisely, I found it very easily. Some of the questions I was asking myself have been answered now. For instance, when we see Frank coming out of the hill to go to young Harmonica, does that hill is somewhere far or near the arch? Well, it's exactly the same location, placed like it's supposed to be in the film, I mean, that hill could have been a few miules from the arch, but nope, it is really in front of the arch. Here's a picture taken on top of one of the base, showing the cement tracks for the back travelling sequence and, in the background, the hill that Frank came out.

The arch has been painted cause in the movie, we can't see the cement between the bricks, but with time, wind and natural effects, the arch has been kind of "sand blasted". The beam that we see in the next picture is not a beam thrown into the arch like that. It is part of the structure. The arch has been made of wood, then recovered with bricks.

Here's the inside of the arch:

The Arch is located on a hill and can be seen from up to 4 miles (without binoculars) Leone chose this location for the nice background of monument Valley we can see. This next picture is taken with a 10X zoom:

The central part of the arch had collapsed in the mid 80's according to Frayling's book. Here I am with the arch, nice souvenir for me:

If some folk on this forum has pics of the arch before it came down, please let me know and send them to me, this would be very much appreciated. I just wish I could've seen the arch complete.
The location