Agree with Clifford's solution. You may get lucky enough to get a CS tech pick a good one for you.
Otherwise, its' time to get a Forster Co-ax press. The machined flat bottom under the universal shell holding plates is always parallel with the die holder slots (i.e., perpendicular to the die).
It occurs to me a lot of complaints about case heads not being square to the rest of the case body may be caused by the shell holders. I've found the identifying number stamped in the top sometimes has raised edges, too, that I flatten. It's not hard to set up the lathe and an inside toolpost grinder to square these little guys up, but you shouldn't have to. I also wouldn't be surprised if warping during heat treating is the problem.
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