I just replaced the heater core in my 87 GN and I would not bother putting in a used one as the aftermarket one's from places like Napa are nice and only like $26 with warranty. You can still get the GM one as well from several vendors but they are like $50 when I was shopping around. It was not the easiest thing to replace the heater core due to age and the RTV used to seal everything in. Make sure the ground strap connected to your old core is actually a good ground where it attaches to the firewall - mine was in bad shape and may have contributed to the corrosion failure of my unit in spite of very regular coolant flushes, etc..
Good luck
Billy