Your assessment sounds plausible. I also agree that the
HST's are probably the most robust transmissions you can get.
Your $5500 gamble looks like it will pay off nicely for you! Were you able to get a used back half or is parts and pieces cheaper? I've found that on 30+ year old iron, used whole assemblies are often much cheaper than repairing what's broke.
Good luck and keep up the very informative posts.
jb