Hi Rob,
I am running the same chassis as you know, with a 65 mustang rad (fits between the rails on the tilt). I have the rad lower than the outlet from the engine. It doesn't seem to worry the car in the testing I have done. It doesn't blow water, or anything weird, which I thought it might. I have my thermo fan set to turn on at 85 deg, and I struggle to get it to turn on (I set it low to know the temp of the car, I'll lift it again later). General cruising I don't use the fan at all (I run an
oil cooler and a trans cooler also), the PCM doesnt turn it on. Only 'fanging' it around can I get the guage up, and the fan on, but its more like a race situation, first gear 5000rpm for a long time. Even in heavy traffic on the ring road (20kph for an hour) still only just get the fan on, I think I remember 87 deg. The moment the speed picks up a bit, its off again. So I wouldn't worry about it.
Remember that the radiator position is related to the shape of the car. If it was an issue, it would be an issue on almost all replicas and the real ones too!
Give me a buzz if you want a chat about it.
Nath