my 390 inhaled metal piece. need help assessing extent of damages

drove a buddy home other night bout 10 mile drive i was getting on the gas pretty hard. but on the speed limit abiding trip home i was takin it easy and then suddenly developed what sounded like a rod bearing knock. luckily i was only about 5 blocks from home so i pretty much coasted the rest of the way home. Revving the engine a little would cause the knock. i kept it under bout 1600rpms n nursed it home. Got home pulled pan/heads and found passenger side rearmost cylinder (#4 i think) had multiple impact damage on piston, some very light impact damage on head but the sparkplug electrode was smashed into and contacting plug. surprisingly there was no visible damage to cylinder walls. there was a 1/8" chunk of magnetic material fused to the piston head from getting smashed between piston had head. i have no idea where material came from and how it didnt make any noise while i was rippin around town. found no other damage, like i said i was pretty much coasting the last 5 blocks home when knock developed. the aluminum piston head i think took most the damage. the engine had maybe 4000 miles on a rebuild done at one of the best performance shop in town. They specialized in FE motors, so i knew it would be done right. so the engine is in good shape otherwise. i got a new rod bearing which didnt show any damage but i dropped n bent bearing so i replaced it. i wanna plastigage the rod bearing; should i guage that with bearing dry, oiled or oiled and greased? cuz i got conflicting info in some books. also what is the plastigage tolerance on rod bearing? any advice or feedback would be appreciated. its a 69 mach 1 390 4spd. thanks.