as has been said, the clips of the lifters got out because you had lash at some point during engine operation, allowing the `cups` of the lifters to contact the retaining clips. i looked in the speed-pro catalog (sealed power), and for the anti pump-up lifters they advise setting lash at .002in. HOT. they also recommend that if the engine is noisy at this setting, that you should tighten the lash. after seeing what this approach did to your lifters, i would lash HOT for a 1/8 to a 1/4 turn preload. i would use NEW lifters, same brand ok. as for installment precautions, while the lifters are out of the engine, i would directly pour thick 20w50 oil on the cam lobes through the openings of the lifter valley and use assembly lube on the base of the lifters before installing them in their bores. for break-in, use the same procedures that would be needed with a new camshaft i.e. 20 minutes at 2000rpms, then change the oil. during break-in, notice if the engine is noisy (valvetrain), if so, tighten the lash a bit AFTER the break-in. this time, you should not have a problem with your lifters again.