Chris,
3-3/16 inch center distance. The distance piece body is slightly shorter at 3-1/8 inch and is 7/8 x 1-1/2. I have been meaning to do a drawing of this assembly, so hang tight.
Also, without looking up your other posts I forget if you are doing a 1934 or 1935 model, but 1935 was pretty much the year Douglas changed over from one inch to 7/8 inch handlebars. Which changed the distance piece and top clamp components accordingly.
-Doug
[fix typo. 10Jan22 -Doug]