If its an A/C box the box itself has threaded studs that poke through the firewall that attach with nuts on the engine compartment side of firewall. There are 3 nuts around the blower motor then 3 or 4 more nuts kinda evenly spaced across the firewall from the blower motor area over towards the drivers side of the firewall. Then there is a hanger support bracket on the dash frame that connects to the front of the box behind the glove box insert. I just went through replacing a heater core in an A/C box on my 70 Cuda and it also took me a bit to figure out how it mounted. Good luck!