WI needs the measurements between the centers of the heims. I have a custom linkage, but the style and the heims are of a style different from Chopper Dave's. The contact at WI was Aaron. If he is still there, he is the guy to talk to. On my linkage, my heims were installed in the linkage, at right angles to the linkage. You will note that Dave's are installed at the ends of the linkage. I think mine looks "cleaner," but there is no adjustment available for shift linkage length, and you have to be "spot on" when providing WI the measurements.
Chopper Dave is the guru on this shit...he helped me when I was looking for chrome spacers and hardware to get mine mounted.