If they are edelbrock victor heads the springs, keepers and retainers have to be purchased and installed later. The keepers pulling through the retainer would seem indicative of Cheap Chinese parts and or springs going to coil bind and that causing the damage.

Your lucky a valve never dropped or snapped the stem off and thus dropping anyways.

I think you have lots of checking and work to do to make things right you may as well start with premium rods and pistons hone balance and go from there.