I got the perches marked up after finding a water glass that was a perfect 3-3/8" diameter.

I ended up just tracing the perch out on thick paper with it's existing 2.75" arch in it. Then with some measuring, I got the water glass positioned over the paper in the right spot, maintaining the top of the original arch. Then I traced the new arch into it. Then I cut it out, eliminating the line, to allow for the width of the pen making it bigger when tracing earlier.

I walked it out and checked the fit on the axle tube and it looks good.

Now, knowing that I probably didn't get it perfect, I put a 1 on one side of the paper and a 2 on the other and then I made the same marks on the perches and went ahead and marked them for driver and pass sides too.

Now I marked out the new arch with a silver Sharpie. I used the 1 side of the paper to mark the 1 side of the perch and then I flipped the paper over to the 2 side before marking the 2 side of the perch. This way, if my paper arch was slightly biased to one side or the other, it kept the bias the same on both sides of the perch when I marked them.

Pics:
https://www.dropbox.com/sh/6ojbmgdgwzo6d0d/AACPQg-uE2Bafu2Iw0jYOxvBa?dl=0

Larry W.

1953 3800 Dually Flatbed