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I recently took apart a Zenith that was on my Model A for 28 years and found the inside spotless.
I would imagine over time almost any paint put on the inside of a carb will loosen up or dissolve and possibly clog up passages and jets.... If Henry didn't do it I would not either. No need to re-invent the wheel here.
As far as applying Glyptol inside an engine or gearbox, to me it's a waste of time and money. The only one who will ever appreciate the cosmetic look is the next person to rebuild it. If sand castings haven't come loose in 80+ years I doubt they will now. But hey, your money, your car, go for it !