IMM Engines 408 build

Understood.

The only way I can think to check them is to put oil in them and put a few hundred lbs of pressure on the plungers, and see if they hold. But since the ones you already have seem to have been good out of the box at IMM, then even that is not a guarantee.

In the old days, it was common to pre-fill lifters. The best explanation I have seen for not doing it with high lift cams is that it can hold the valves open and slam a valve into a piston. Since you probably have a high lift cam, that seems to be the wise course.

So IMHO that is 2 strikes against doing anything with them up-front. Maybe disassemble and clean them out thoroughly?

The good news: Your high frequency hearing is still good!