Another dozen listens and my final take is:

          I once knew (new?) with lots of tin,
          I never thought (I? it?) could get so thin.

Can't tell if "knew/new" goes with "once" or with "with."

My personal almost-final tentative educated SWAG is that James Wright was singing,

          I once was new with lots of tin,
          I never thought I could get so thin.

Snuffy, thanks for "tin." Let's try another one!