Just a bit of an update.
Turns out that the bolster was broken. Not unheard of for the large frames, but is IS something that he should have been aware of since it was so visible:
Also, while the redo of the seat was a good job, I didn't mention that there were a lot of pinpricks on it. It puzzled me for a bit when I picked it up, but when DeepWrench mentioned it I remembered that Ben has 3 cats roaming around at his new place. Seems like they took a liking to the seat. Not the end of the world and they didn't go all the way through the material. It's just a bit unsightly.
DeepWrench says that he has an OEM muffler and we are going replace the 'stack' with it so that it will be stock when he's finished.
One of our members, Sam, was good enough to take pics and measurements of the muffler support of his HDT for us so that we won't be guessing as to what it should be.
Looks like things are coming together.