Having pulled a 5/16 nut out of the piston on a 396 Chev. I feel for you. I believe you have exhausted everything you could do short of an engine pull. There were 1 couple idle jets laying in the area just above the intake manifold, under the tin, on my last engine. I could see them so I was sure they weren't in the engine. HOWEVER, Just as Stan said, be careful the next time you remove the plugs. This is not very reassuring though. If it were me, and I couldn't see them, I would keep going until I found it. Why ? It wasn't fun pulling that V-8 apart and replacing a piston.
"There is never enough time to do the job right the first time but always enough time to do the job right the second time !".............Bruce