The machinist I use told me his supplier refuses to sell him a flat tappet cam because there have been so many low mileage cam failures. If a customer wants a flat tappet cam in an engine he's building, he assembles the rotating assembly and heads, then gives them the incomplete engine to install their own cam. He has a row of chewed up lifters on a shelf by his front counter that he pulled out of low mileage engines. Cheap(Chinese, mostly) offshore lifters are a problem, poor break in procedures are a problem, and modern oil formulas are a problem. If you have stiff valve springs, replace them with light springs for break in. Run the engine for 20 minutes while varying the rpms between 2000-2500 with high zinc oil or a zinc additive, then change the oil and filter with more high zinc oil. Then reinstall the stiff springs. You can either buy racing oil with zinc or use a zinc (ZDDP) additive if you think cheap oil is a way to save money. By the way, it ain't.