Sounds like icing. I'm not familiar with that engine, but the symptoms sound like a problem I had years ago after I put a header and an aftermarket air cleaner on a 4-cyliner. I've never had the problem on any other engine/vehicle, but it sure was a hassle when it was cold & foggy.
Can you rig up a shroud around exhaust manifold and a tube to the air cleaner? Didn't these engines have that kind of thing originally?

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If you can't fix it with a hammer and screwdriver, you need a bigger hammer.
1965 Chevy C10