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The consensus is get new heater hoses, and a GM replacement core. Many post of Napa or Auto Zone units not fitting properly, do a search on heater core, there was a run on them on C4's about 2-3 months ago.
Take your time, take almost all of the dash out that you can, especially the right side, and be patient. It was a religious experience for me. Took all day just to get to it.
Re-read Renoman's and BILLSF's answers, they speak the truth. I did mine on my 85, that's how I got my avatar but the 85 is different than yours. Justardnck did his, maybe he will chime in here or you can email him for advise.