Fork legs
To keep my triple trees from being to wide, I like to make my fork legs slimmer. I cut my tube down the center about 4 to 5 inches and remove the top half. I then use a length of 1"x1/2" flat stock and lay it in the opening and weld it in place so there is a nice flat surface to mount to your rockers. Clean it up and then do the same to the other leg. Once those are done to your satisfaction, line them up together and drill a 1/2" hole to bolt to your rockers. Make sure both your legs are drilled the same.
Now bolt both your fork legs to your rockers. Bolt them up snug in such a way so they can move but not be sloppy. Now you can measure the width of your total rocker and forks, to determine what the width of your trees need to be. If you like, you can skip this step and simply bolt your legs to the rockers without cutting them down this way. It will just mean your trees will be a little wider.