Dont give up just yet.
I would remove the vent tube from going to the bell housing even if you have to cut it. You can easily splice it back together, I would try to spray brake cleaner into the vent tube going to the bell housing to flush out any oil that still may be in the vent and in the bell housing area, this should rule out the vent as the leak.
Just some info about the vent system on that trans.
This trans uses two vent ports on the top of the transmission one on the rear of the case and one about half way up, they are run together with a "T" at the location about half way up.
There are reports of fluid being forced out the vent tee from the shell that spins in the case just under this vent, Ford and rebuilders remove this vent to prevent leaks from the vent into the bell housing
If this is your case and the vent is leaking you can connect the vent tube from the bell housing to the vent in the rear and by pass the vent tee in the middle. Seal off the tee by using a rubber hose looping it from one side to the other on the tee, it might look crazy but it will work, there just isn't enought room to remove the vent and seal the case.
You can see the vent tee sticking up on this picture.
This a picture of the vent tee removed and blocked from the factory.
