I'm having the damnedest of times trying to get this heater core in my car. The heater box is .. i guess "stapled" into the fire wall with these circular button things. Has anyone ever ran into this before? It came from the factory like that.

It's a 66 Chevy Impala 4 Door with A/C & Heat. I'm sure any chevy in 65 & 66 with A/C and Heat will have a similar setup. Has anyone ever delt with this? It's such a pain! I can't get it to come loose! I've taken out all the screws.