Mike, I'll pass along something that I've been told but never tried. According to the story, the cylinders have a felt type seal that could be tightened to reduce leakage by turning the nut. Can't agree or disagree with this as I've never repaired one. The book shows a series of 5 washers - probably felt. So tightening the nut may help. My guess is, the nut is bottomed out and it won't help.
According to the manual, it is as easy as you suggest. I suspect getting the old ones out may be a bit more difficult. Be careful not to scratch the ram or the cylinder. The ram might push out the packing - don't use hydraulics to do this. Putting in the new one should be easy as well. Be careful to put it in straight and the correct order. Allis may have switched to a rubber seal with a metal frame. In that case, you may need to make a seal driver out of a pipe. If the replacement are felt, re-installation is easy.
Inspect the ram and cylinder carefully. If there are scratches on them that you can feel, a new seal might leak as well. In that case, replacement of the entire cylinder is in order.