Which Catalytic converter do i replace?

I am trying to get my 1988 Ford Bronco 5.8L to p smog in California and it has failed twice and i was told that the catalytic converter might need replacing because it was the same cat on there since 1988, so when i call up for prices at local muffler shops they all are telling me that the Bronco has two cats, one in the front and in the back. In order to p California smog test, which cat should i replace, the front or the back?