I don't know the amps- I owned ARC in the early days. The company does (or at least used to) have authorized repair facilities in different cities. I believe Pyramid in Austin is one such place.
I really don't like shipping tube gear, so assuming you want the amp recapped and gone over-- have no idea what "mods" you contemplate- you might reach out to Pyramid and take a drive with some muscle to help you.
I have a fellow I use here for vintage tube gear- his main thing is antiquarian (pre-WWII) and he is especially good, but will not work on anything that isn't pure tube stuff- no hybrids as far as I know.
Back in the day, I did have ARC in Minneapolis do some mods to my SP-10mk ii. Those were factory sanctioned and because I was offering to be a crash test dummy for a couple things- like gel tube sockets, they were willing to accommodate. Haven't dealt with the company in a long while so can't speak to their current policies re "mods."
Best of luck,
Bill Hart