You'll need a micrometer and exacto knife.

First take measurements. Then do the calculations to see what thickness of shims are required. Remove the discs and see if you can switch them around so that only a shim or two is required.

Then find some foil of suitable thickness (eg. in kitchen), cut it to size and place it under the disc(s) that needs thickening.

NOTE: in removing (and installing) the cam shaft, loosen (and tighten) bolts/nuts in a balanced way, else you may snap the shaft.
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1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.