I would never use a flat tappet used cam in a different block or if the lifters got mixed up. The get broken in to the block, every lifter bore has very tiny differences in lifter bore angle and placement over the lobe so once it is broken in you will probably not get at least one bore close enough to the same that it don't wipe out the lobe. People think it is just a matter of keeping the lifter on the same lobe but it is also about the bore being centered or angled precisely.