Flat is OK, as long as you don't intend to run it wide open with a heavy load for hours at a time. Yes, there will be a little cam wear, but that's why you always keep lifters sorted out by cylinder number and position and run them against the cam lobe they were on to begin with. Swapping used lifters around onto cam lobes they haven't been running with guarantees a very short lobe and lifter life.

Cupped lifters are a sign of a totally worn out cam and lifters. Time to scrap the whole assembly in that case and start over.
Jerry

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