Hi,
I don't know for sure but, if you have the old one, it might match up to something they have on the shelf at NAPA for old GMC trucks. Or, just get a later, skinnier one and bush the hole with a brass adapter. Those sensors were NTC carbon and were pretty common back then iirc.
As to your alternator question in another thread, I don't know that answer either. I haven't worked on a 6-71 since the 90's and, even then, the in line engines weren't really used in industry very much and reusable parts for the V engine were getting pretty rare.
You might try to find an old bus boneyard. They might have what you need but I'm guessing they're going to want some serious money if you can even find a match.
I think I would first look into maybe finding someone to rebuild the thing for me.
Sorry I couldn't be of much help.
Good luck.