Roger that! Remember, the ZF was originally designed to run upside-down in the GT-40 and Mangusta. So if you pull that critical bolt on those cars, the 4" long extension shaft slides outside harmlessly. On a Pantera, it slides inside and there's just not enough space for it. I've had to weld ZF cases back together and replace chipped gears when this happens- $$$$.
To fit an angle drive, you screw the nut all the way onto the gearbox fitting, fit the angle drive to the double-thread nut, and turn the nut back onto the angle drive until it bottoms. That maximizes threads on the angle drive and positions the drive tabs correctly so they won't shear. Leaks are take care of by o-rings, not how tight the double-nut is.
Second, the original grease inside is rock-hard after 50 years. Some guys soak the angle drive in solvent, then in light oil for a week or so. It's also possible to drill & tap a hole in the angle drive body and add a Zerk so the pricey assembly can be serviced now 'n then. Or use an electronic speedo and eliminate the angle drive and drive cable completely as DeTomaso did in the Pantera Si.