Probably too late for this, but check the gasket inside the two halves of the collar. I've seen a couple of weird "farmer fixes" (that didn't work) done in this area to stop leaking. In my coupe that I just did, someone put rope seals from an 8N tractor. How do I know? Simple, I had an old (NOS) pair of rope seals from my job and they were the same length, bout a 1/4" too short! No wonder it leaked there! Proper ones are felt and a 1/16" too long and have to compress a little when the two halves are tightened. My other coupe had a homemade "shroud" soldered around the two halves. It really wasn't clear what that was supposed to do. I estimate it took at least three hours to get that thing on there and for what? The felt seals are a heck of a lot cheaper (Under $3. from Brattans) Even a cheapskate like me can see that.
Terry