What I do on the Camaro clip.Is before you cut the back off,Set them level,Once they are level,Tack a piece of angle iron on the center of the crossmember ,Front to back.About 6 to 12'' long..Make sure when you tack it,It is also level.Then you have a place to set your level.Put that level, In you won't have a problem.Then you can cut the back,And start fitting it.

This is how I do all the camaro's clip's I do.(If you are using a Camaro clip)..Hope this helps you out.. :thumbup: