Re: 1940 Farmall M belt sizes, need help.
I just took my belts in with me to measure up against the new belts and an auto parts store.
I have an alternator on my B414 though (don't think it's original) and the belt that was on there was a little too long (adjustment was out to the end and the belt was a little too slack) so I bought a shorter one.
Of course if you want to order them online it might be an issue but since you still have a generator, they should be the same size as the factory originals. Maybe call up a CIH dealership and see if they have the sizes against the part numbers.