Hello Iain
Interesting thread for me since I have been following the aerodynamic GT40 revisions since the first Ford announcement way back when
I am not an aerodynamicist, I'm an architect and the only wind tunnel experience was dealing with wind loads and flow on high rise buildings (example: 1,135ft high), where the wind speeds are in our area of concern, roughly 200+/-mph
Enough of that nonsense! and a little history (as I recollect it)
When the GT40 was first announced there was a lot of literature regarding aerdynamics since Ford had done a little work with the Lola GT coupe that had wind uplift problems at the vehicle's rear at 130mph, and Ford was expecting to go against the Ferraris that didn't have this problem at 180mph
Ford's initial model research indicated that the GT40 would become airborn at about 230mph so they thought that they had the problem licked
On the track however, things didn't match since the rear weight bias transfered the lift to the front, and they tried a lot of variations
The nose was kept at a half to 1 inch lower than the rear and the nose itself went through a lot of variations
One problem with the nose was the 2 nostrils required to get airflow around the spare, unrecognized at that time was that the air was flowing around the cockpit rather than optimally over the top
The single nostril allowed air to flow up and over rather than around and allowed this down force to augment the nostril downforce and apply more force at the windshield, closer to the vehicle cg
Later a center divider was added in the wider single nostril to better guide the air over the roof
When you look at the Mk IV, you will notice the raised side fences after the nostril to again force the air over the windshield
The rear had the "duck tail" to add some downforce which basically was adequate at high speeds, a rear adjustable spoiler was added but used mainly for short tracks and rised the duck tail by about 3/4inch, full extension of the adjustment wasn't required
The small wings ahead of the front wheels were tried but usually abandoned since their effect was nil in comparison to the nostril modifications
The Lola GT had used openings in the rear glazing to relieve underbody air pressure but on the GT40, this negated the cool air entry from the after door scoops that captured cool air, not rather warm engine compartment air
The tail's openings alongside the central exhaust exit were used to relieve pressure build up and enlarged on the Mk II which had a slightly greater speed and engine heat from 100hp additional
The GT40 MkI body is pretty good aerdynamically and a wide single nostril with central divider and adjustable rear spoiler, and keeping the front slightly lower than the rear, should do the trick
A wing would produce more than adequate required downforce and a lot of drag, it shouldn't be required
The rear diffusers exhausting through the opening alongside the exhaust opening would be an interesting experiment if downforce on the places where you're accelerating at the max is causing wheelspin, might more easily be solved with a wideer tire tread
All of the above is personal conjecture and recollection and I hope that our group will tear it apart
Whatever you do, keep records of what you find and post them here
I hope that this doesn't bore everybody to death and hope that it might provide some help in your efforts
Best regards
Dennis