Good topic, this is something I'm worried about myself.

Apparently this has been an ongoing problem with the originals also. Looking at P1021 in John Allens book they fitted large canard fins to the front for downforce. And this car had the large single rad outlet at the time. This was during a vintage race in the '90's.

Just by intuition I would confirm what's already been said. Lower front and/or raise rear ride height. Should be about 1" difference between the two. And also look at internal (front) aerodynamics.

Most likely the rad intake is more effecient/higher volumn than the rad outlet.

One thing I have noticed is that the spare tire cover (body piece that includes rad outlet) is not exact on some of the replicas. I have an old KVA body and the ducting of the spare cover does not go all the way to the radiator. In fact it is about 12" away from the radiator. Looking at the spare cover on a MarkV I noted that the ducting goes all the way down to within a 1/4" (flush fit) of the radiator with very smooth lines.

Without some type of added ducting the KVA spare cover will be inneffecient and might cause a lift problem.

Another thing to think about would be not enough anti squat in the rear suspension. Imagine going 70 mph and slamming the throttle. What if the car squats hard and adds to an existing front end lift problem?