If I was starting from scratch and having to come up with a shifter arrangement for a ZF, I wouldn't re-invent the wheel. I'd just get the entire shift setup from a wrecked Pantera (lots of them are available if you know where to look) and plop it right down into the car.
Here's a photo of the shift lever and housing; it's a beautiful aluminum casting.
And here's a diagram from the parts book:
(Front of car is towards the bottom of the image).
Here is what happens downstream:
The nice thing is that this whole system is a SYSTEM that was engineered to work properly. Although in the Pantera application the shifter box on the gearbox is on the left side, it would be a simple process to re-route the linkage to go towards the right side for a GT40-style application.
If anybody is interested in such a setup, let me know. I'm going to look at a wrecked Pantera this afternoon, and I know of at least a half-dozen more in Ohio, and more in Connecticut....