Where is the breather tube connected? If it's on the 90 degree elbow on the steel cover, then it's still very possible to spit out even with the gear
oil level being 1" below the fill plug.
What you want to do, aside from making sure your level, viscosity and anti slip additive are all proper, is to PLUG the breather elbow on the steel cover with a stout rubber plug and rubber cap and clamp it. Then remove the tapped plug on top of the diff and install a 90 degree fitting (I may have an extra if you need it). Connect a hose and route it to the high catch can you mentioned you have in the trunk.
Once you have done all this, go for a drive and monitor the can, especially as you really heat up the diff.
Where the diff is breathing from CAN make a huge difference, believe me. I had this same issue on my CSX roadster (Kirkham diff). PM me for more detail on that.
Good luck!