I am not sure what you mean by “reproductively stable genes”. Do you mean that some genes are ‘less likely’ to mutate? In that case no. Genes are all all equally likely to mutate. Furthermore, yes, further mutations could cause another increase in neck length. But at some point, a long neck becomes a disadvantage, for example if the lever principle causes the animal to fall over constantly, or if the heart is not strong enough to pump all that blood to the head, making the animal tumble around all the time. In that case, a long neck would be selected against. It is not that less mutations happen, it is just that a further neck length increase would be selected against.