If it were mine I'd probably be running a quality conventional 10w40 High Mileage, unless the engine has been recently rebuilt, then skip the
HM. Non detergent oils allow gunk to build up instead of being removed when you change oil. Oils have been vastly improved since your tractor was built. why not take advantage of that.
I'd do a bit more research before feeling comfortable using a 20W oil in a 60+ year old loosey-goosey engine. Modern engines can use a multi-viscosity or straight 20W oil because they are made to run those oils with nice & tight engine tolerences. Your 64 year old Ford, not so much! That's why I initially recommended a 10w40 HM oil. At operating temperature it's a 40W oil and the HM will have seal conditioners to help keep those old seals a bit softer.
A modern 15W40 or maybe 10W30 Diesel rated oil will have the additive package and viscosity that an old 8N can benefit from as compared to a modern day S- rated gas engine only motor oil. If you want to use a single viscosity oil I 'd use nothing less than 30W and certainly NOT a non-detergent of any type.