There was never a gasket between the manifold and head on 283, 307, 327, 350, 396, 427 in the 60's. Somehow the aftermarket created a demand for those. Those engines I mentioned never ever had issues with leaking between the manifold and head unless there was an issue with the manifold being warped or the mating surface being bad. What I have seen happen a number of times is you put a gasket in where it's not needed and when the gasket burns out (which often happens) is the manifold either warps or the mating surface is damaged and won't seal properly after.
A nice long sanding board with nice coarse sandpaper to clean up both surfaces, bolt it on with no gasket and Bob's your uncle!
I think people often put them in just because they come in the jobber kits.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars
I don't recall the last time I ever saw an old carbureted Chevy V8 leak between the manifold and the head unless there's a crack, a broken bolt or some obvious issue.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars