Good deal. They actually make tools to install the rod seals that can help. I have found a little heat from a heat gun will make the seal softer and easier to install.
You should be able to do it in place. Like you said take the clevis off, take the hose off then unscrew the head with a spanner wrench. You will have a rod wiper (the seal you see from the outside), a rod seal, possibly a rod wear band and a outer head oring and backup. Just pay close attention...
I purchased a TH400 from PTC maybe two to three years ago. At the time Ed would not build it with a compression braking vb for off road. Maybe they didn't put one in it?