Ken, This is a big job in a full size GM van. I believe the entire dash has to be unbolted/screwed and propped up or removed to access the heater core for removal. These instructions are from 2carpros, JACOBANDNICKOLAS
Disconnect the battery ground cable.
Remove the engine cover.
Remove the steering column-to-instrument panel bolts. Lower the column carefully.
Remove the upper and lower instrument panel attaching screws. Remove the radio support bracket screw.
Raise and support the right side of the instrument panel.
Remove the lower right instrument panel bracket.
Remove the vacuum actuator from the kick panel.
Disconnect the temperature cable and vacuum hoses at the case. Remove the heater distributor duct from over the engine hump.
Remove the two defroster duct-to-firewall attaching screws below the windshield.
Under the hood, disconnect and plug the heater hoses at the firewall.
Remove the three nuts and one screw (inside) holding the heater case to the firewall.
Remove the case from the van. Remove the gasket for access to the screws holding the case together. Remove the temperature cable support bracket. Remove the screws and separate the case. Remove the heater core.
Will post a couple links to Vid's and to a Van forum, to show what you could be into if changing the heater core in your Chevy/GMC van. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
https://www.youtube.com/watch?v=XIavJiKhMHo
https://www.youtube.com/watch?v=Fv2Yb4xADx4
http://vanning.com/threads/ubbthreads.php/topics/533063/all/Heater_core_GMC_Chevy_vans.html