[Mart]

Personally I have used single circuit cylinders in my cars in recent years. Coops spells out the issues in his post above. Retrofitting a cylinder to an old fashioned car like our early fords is probably not a great idea.
Repeating what coops has already said, dropping to a 1" cylinder will mean you need all the available travel. When a dual circuit brake system fails on one side, you need extra travel before the brakes on the good side can be applied. The chances are you will not have enough travel.
I do have a dual cylinder on my roadster. It is 1-1/16" from an E250 van. It is a big old cylinder though, much bigger than the 1" cylinders often used. It fixed my excessive travel problem but I have never tested if it would work if one side failed. I suspect it would not.
I prefer the simplicity of the single circuit and ensure the system is maintained in good order. I also ensure the hand (emergency) brake is also in good working order.
In summary I believe swapping to a dual cylinder is not necessary, may impair brake "feel" and may not give the intended advantage of maintaining some braking in the event of a failure.
Just my opinion.