[Bassman/NZ]

I would suggest the rear main bearing shell is getting a bit worn, causing the new seal to go out of round a little.
What we used to do in the old days when flatheads were so cheap and readily available that we never rebuilt them, just replaced them, was to make up a little contraption out of a tin lid or any suitable metal container bolted to a piece of strap steel which was in turn attached to a bellhousing or oilpan bolt. This caught all the drips, and once a week, month or whatever we'd drain it and re-install. We used a wingnut so it could be detached and drained in a matter of seconds.