We would need to know if this is original dash A/C or under dash.
With all due respect, it can't be that hard to figure out where a large piece like that fits. It has a specific shape and mounting holes. It's either a piece inside or outside the cab. If you don't remember it at all, maybe someone piled a piece on your stuff not associated with your truck. Seems like you would remember that big thing. Does it go on/in outside firewall heater box where the heater core is? That one angled side of the hole does show up on some heater core baffles inside firewall box.
It's also funny because one part looks like a dash radio impression.

It seems like when you put all together that you will see something big missing. I would just move forward on the install and a light will go on.


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn