The Spring perch is designed to be used in one of two ways. You can weld the gussets to the edges of the perch or in the center of the perch. Personally I prefer to go with the center position. It seems to flow better with the rest of the piece sizes around it.
Measure to find the front crossmember. Slide the gussets so they are equal distant from the center. Square them up and weld the spring perch to them. Now set your frame so it is level. Raise the spring perch so it is 5 degrees up from the top of the side rails and weld it to the crossmember tube.
At this point I put a temporary K type crossmember just forward of the kick to hold the dimensions until I get a center crossmember installed. Then you can take the pieces off that you added to hold the squareness before.