How do you replace the heater core on a 1996 Astro Cargo with A/C

Welcome to the forum!
Heater core replacement is a time consuming process in the Astro/Safari van. There are no special tools required and it is not a difficult job however; I would recommend purchasing a shop manual for the vehicle. Most auto parts stores have Chilton and/or Haynes manuals for around $15 USD. The replacement procedure is outlined quite well in both manuals.