how do i replace the heater core?

Hey tman ...The manual is going to be the best, most complete guide to switching out your heating core and can certainly give you more answers than I can. I would suggest double checking your removal procedure to see if you missed either a mounting screw or another piece that needs to come out to remove the core. It sounds like you may have missed something. If that doesn't yield results, bring a 6-pack down to your local Chevy dealer and find a mechanic who is willing to give you a few "free" pointers on your job (it will probably be the best $5 you ever spend). Good luck!