The steering box hits the header both with the shim and without.
sounds as though you are being backed into a corner
With the R valve cover and shim removed does the header still hit? and if so, by how much? how much lift on the on the L side is required to gain adequate clearance? While in that lifted position how level is the carb flange ?
Lot's of questions but trying to establish what is the path of least resistance to correct the issues
