Modification of K-member idler arm mount advice

I think your jig is correct. Notice the lower control arm pin hole on the black k frame. Look at the contour of the top of the k frame in that area in relation to the lower control arm pin hole on both k frames. You can see there's a big difference between the two k frames in these areas and I believe that's where your difference is. I would go ahead and reshape the black idler arm mount where it fits flush on the red k frame all the way around, tack it like Del says and see how close you are. I bet it's right on the money.