Since the ZF is inverted for the GT 40 I do not see how to drain the oil?
My technique would be to use the drain plug on the bottom of the box.
However, that requires that there BE a drain plug on the bottom of the box.
When the ZF gearbox was originally designed, it had a drain plug on the bottom. This carried through with the Dash-1 version as used in the Mangusta, which also ran in GT40 orientation.
When the Pantera came along, the gearbox was inverted, and that drain plug was now on the top; a second drain plug was installed where the vent had previously been, in the top cover (now the bottom cover). The first several hundred Pantera gearboxes were built with leftover parts, so they had the redundant plug on the top. But from early 1972 on, the case was no longer machined to accept that plug since it wasn't necessary.
When a Pantera gearbox is modified for use in the GT40 orientation, one of the critical steps is to machine the case for a drain plug. On my GT40 ZF, the case was actually damaged, and Lloyd Butfoy was able to substitute an early-style with the plug already installed. See below (my gearbox now inverted to show the bottom as it appears in a GT40, with my old case pictured alongside):
Now. If your ZF is already installed in your car, and it doesn't have the oil drain plug, you're totally screwed. It would also cause me to deeply suspect the rest of the work involved in the conversion of your gearbox; after all, if something so fundamental and basic was overlooked, what guarantee do you have that the more subtle nuances were attended to?
There are many, many subtle modifications that are required when converting a Pantera ZF for use in the GT40--and not everybody who does this work is guaranteed to know all of them. Among other things, the cast-iron rear cover has to be machined internally to guarantee an adequate supply of oil to the bearings which support the main shafts.
Rather than going that route, Lloyd was good enough to swap my Pantera cover out for a Mangusta cover, which incorporates a fill plug. The beautiful thing is that it allows one to fill it until the oil dribbles out; this ensures the correct amount of fluid is in the box.
This fill plug is not strictly necessary; it's just a very nice feature to have. If you don't have it, you had better have the internal modifications on the back side, however.
Hopefully you are still 'in progress', perhaps looking at a Pantera gearbox that hasn't yet been converted for GT40 use?