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I have Dynomax ceramic headers on my small block. I needed no additional brackets. Even the a/c mounted on the factory exhaust manifold stud. Quite easy to install. Not sure if your 69 has different brackets than my 80.
I cant answer about the ac, but for the alternator, get an iron mount that goes to the front of the head out of a 72+ or I think the 80's have aluminum units that bolt to the front of the heads. Either one is completely clear of the headers.
Is your alternator on the driver's side, high? Mine attaches to the block and the driver's side manifold (stock). I also have the original A-6 axial style compressor (A/C).
My alternator brackets mounts high at the front on the head. My a/c unit is the newer short style. It mounts to the front of the engine at the water pump and head and has one lower support that mounts to the front bolt/stud on the exhaust manifold/header. Sounds like your car has different brackets. You may be able to find brackets off a newer vette at a vette resto shop that keeps used parts. There are several in my area that have huge warehouses of parts. Cheaper than buying new from the catalogs.