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The only thing you need to do on the 78 is make up a spacer(or stack of washers)the thickness of the original exhaust manifold minus the thickness of the header flange to support the side of the main bracket.
Everything else will bolt right up.
that is correct on my 80 as well. it has threads to lock down the header and then threads on the other end to lock down the compressor. Great fun to work on these cars. They smoked a lot of pot coming up with these workarounds...
i threads into the Head just like the headder bolts. It will be the same thread they have... i don't know what size that is but if you match it to one of them you will be fine. The long part can be any size you want ... should be plenty of car shows this weekend where you could go look at one in person and see what we are talking about...
You could always use a length of 3/8-16 threaded rod.
Screw the rod in in place of the header bolt, slip your spacer over that and lock it down with a nut. Install the bracket and lock that down with another nut.