You've got a lot going on so I'm just going to post random thoughts based on what I see and have read.
The guys who know these cars aren't really the Instagram crowd, they're 45-65 YO guys who primarily use TA/Firebird forums and some join facebook groups and use it a little.
The bumper is likely the 70-72 type and the grills are '72.
The 73 bumper is different construction for higher impact rating and weighs more. A stripped 70-72 bumper weighs 45 lbs IIRC.
You really don't want to sell of the bumper as a "kit". You'll collect twice as much selling off the grills, brackets, etc separately.
The front wheel flares are different so if you got some with the 76 you'll probably see they're different than the ones on the '78.
I don't see the brackets for the fenders that the early lower valance would bolt to so the 76 may have it's original fenders.
Don't break anything or throw anything away because you don't need it. Almost every part can be sold and some that are broken or appear worn out can be rebuilt. An example would be a Formula steering wheel. Someone will buy your crappy old wheel and send it out to be restored while keeping their old wheel on the car so they can drive it while waiting on the restoration process. Same goes for dash bezels, cracked dash pads and so on.
Even little tiny parts you might consider junk like the little horseshoe spring clips that hold wires under the dashboard can be sold. Some parts aren't reproduced.
Some parts are one or two year only and hard to find used in good condition. An example would be the inner fender extensions on the '78. They both rust out but the one under the battery is shot on most cars that were daily driven. I reproduce them occasionally but only if I don't have something better to do. None of the big aftermarket parts places reproduce them. https://lab-14.myshopify.com/products/77-78-firebird-formula-trans-am-inner-fender-extension-panel-left
The rear spoiler ends are different for the two years. The ones on the '76 were used from 72?-76. The center sections were made differently but appearance and mounting of the centers is the same from 70-81.
To do your 78 front end conversion it'll be easier to separate the whole front end on the rolling subframe. You'll literally part the car in half and bolt the '76 unibody onto the '78 front half of the car.
Even if the 76 was also an AC car originally you'll notice the engine turned dash panel is different.
If it's still good I'd keep the wiring harness from the '76 with it, including the under hood section. Also save the underhood wiring from the '78. Then you can splice the two harnesses to get the different headlights set up.
Clean off the passenger side of the '76 rear axle on the front side of the axle tube and get the codes stamped into it. Then you can find out if it has a posi unit and what gears were factory.
Few pictures of the 76 T/A 
