Help! Can't get Paragon P23L gear case apart

[sailingunity]

I had a problem with my prop dragging while in neutral, and per the recommendation of both the manual and Basic Power, embarked on a journey to replace the clutch pack for my Paragon P23L transmission.

I've got the gear case out of the transmission, the set screws are removed but I cannot get the end cap to unscrew from the gear case housing. I've even built a homemade spanner wrench with a 18" long handle, and still no dice (I actually ended up breaking the jig that was using to hold the gear case from rotating).

In the attached diagram, the parts with the 2 arrows are the parts that need to come apart. I'm convinced I've made some glaring error and there is some simple thing that needs to be removed to get them to unscrew, but I cannot find it. Does anyone here know?