Very interesting car indeed, and certainly very different.
I'm curious about how you are finding the ZF gearbox is surviving with the torque and rear grip levels the car has?
I earlier ruled out using a Sunbeam Lotus ZF gearbox in my Sunbeam which is being powered by a BMW M62B44 V8 with very similar levels of power and torque after doing some research and taking the recommendations of a well known specialist in them that highlighted that this gearbox would be operating a good bit above its torque capacity if mated to the BMW V8, so I opted to use a 6-speed M5 Getrag unit instead.
Given that these ZF gearboxes are now getting really quite expensive, being in some demand for historic rallying (and their spares and internals are fairly pricey now too), I decided to sell mine and put some of the funds raised into buying the new M5 box instead. Saying that, I believe many of these gearboxes may now have had their original internals replaced with the gearsets intended for the GP4 Escorts of the same period which may be up to a fair bit more punishment.
It sounds like your car has been in use for some time now without any sign of any issues in this area, and although my car will be about 25% heavier in full trim it will also have less dry grip, which should be much kinder on the drivetrain. Also, as my car is only for general road use, I'm now wondering if I have done the right thing in being so quick to discard the cars original and distinctive dog-leg first ZF gearshift pattern.... which would also have avoided cutting in a considerably bigger transmission tunnel to clear the M5 box?
Maybe your gearbox has already had the internals swapped? When comparing the two I couldn't help noticing thsat the stock SL version has quite a spindly-looking input shaft just behind the clutch spline.....