After many diagnostics from many different 'experts', some saying big end bearing, others saying lifters, piston pins et al. I finally found an elderly gentleman in Selmer, Tennessee who had worked on these cars for most of his life and he suggested we check the flex plate or flywheel. Sure enough we found a crack and the offending part was duly replaced ...... Voila ! no more knocking sound.
Just wanted to post this result as a help to others.

Hmmm....if those "experts" had read the service manual, I'm pretty sure it suggests inspecting the flywheel for cracks as a possible cause of noise!

Glad you got it fixed without having to tear the engine apart!

Was this a windowed or solid flexplate?

All 472 and 500s had solid flexplates. I think it was just the 425 and 368? that had the windowed ones. But I do hope he replaced it with the correct one!

I'd like to know where you got your info that the 500/472s are all solid. I have a 472 in my posession with a windowed flexplate, I've measured the stroke and I know it's a 472. It could have a different flexplate, but that would be pretty odd if it did.