I suspect that the push rod was either pulled out by mudbog or, gad, left out!!

Did the adjustment back off - or - are two cam lobes now no longer lobe shaped but wheel shaped??? Common occurrence with some 350CI '70s - '80s Chebbies. The fact that the zinc that helped lubrication of camshafts is no longer an oil additive has to make it worse. Only some diesel oils and Amsoil have a zinc additive now.