When parted out my 2Gb Spyder, i don't recall having issues removing heater core. I'm pretty sure all heater cores are likely same for all 2Gs
However, everything was removed at that time including blower, ac, etc.
You may very well need to remove more parts. Since it's mostly plastic, you could push/pulls different mounting points on the core and it should lead you to tight spots that likely have another bolt that you cannot see.

1996 Eagle Talon TSi AWD
2006 Mazda RX8 6sp TURBO
2014 EVO X MR