Heater core in a 1997 S/T model GM truck isn't that hard to replace. If I remember, warranty only paid about 4.2 hours.
You will have to lower the steering column. Then, remove the IP, dash, radio & climate control head.. Its not as bad as it looks.
If you have SIR (air bags) follow the proper steps to disconnect and then reconnect any wiring.
You will need a set of Torx bit screw drivers. Have a clean space next to the vehicle so you can lay all the parts you have to remove and, take lots of pictures to help you remember how to reassemble it.
Don't use stop leak. It will just make things worse in the future.