Help with removing main ductwork from firewall

Ok first thanks for the info on the blend door and gasket sets.

Second... This one has me thrown for a whirl. The main duct here has screws and latches like it should come apart but then has rivets towards the driver side


The driver side has a bolt securing it to the firewall, but above the heater core is this rivet.


This is where I'm at and about ready to drill out some rivets 😂 which can't be right. So what am I missing?

following this...

and not sure why those had to be riveted  makes no sense...

My blower motor assembly on the 78 was riveted as well. After the motor was replaced, I used nuts and bolts to make up for those rivets.

in order to remove the interior duct air handler, you must first remove the under-hood box. the interior portion has studs fitted to it which slip through the firewall and then the under-hood box slips onto them and then the nuts go on to sandwich the assembly. two 7/16" nuts are between the blower housing and the fender. awkward to access but doable. another is between the blower and the evaporator in a bit of a valley near the heater hoses. the fourth is accessed from the inside beneath the heater core as a 7/16" headed coarse screw that reaches through and grabs the outer box. use something like freeall, pb blaster or deep creep on the studs/nuts under the hood. let stand a bit. this will reduce the stress on the fitment of the "cross" shaped studs fitted into a plastic housing.

Dang I was hoping that wasn't going to be the case. It was so hot outside this morning the grass was burning my feet and I couldn't get under the hood just yet. It's closing on sun down I'll be able to go digging soon. I did get the long screw from under the heater core inside.

I just came in from the garage and remembered that its five points not four. two by fender, one in center under hoses, and one right above the passenger side valve cover. it is so much easier to do hvac system with the passenger fender removed. in my case its almost worth it. but I normally wait on the front fenders for when the cab is complete.

I got the two by the fender left. I'm going to use a flexible socket extension on those it started to rain so I buttoned up.