Hi Brian,
No,I haven't sorted the overheating issues yet, as I plan to change out my heater matrix shortly.
However, after much discussion and research, I have decided that it is down to not having enough air through the rad at higher constant speed.
I have just ordered a front splitter for a transit custom (£109 from EBay), and will look to see if this can be adapted to fit my car, with a return curve to help force more air through my radiator.
Also, after speaking with the GOD of extreme kit cars (that's you JD, don't deny it lol), it was also a good idea to remove the A/C and condenser rad, allowing easier flow of air through the rad.
I can always open my windows if it gets too hot.
I am currently finishing building my ferraris new home, and got fencing to do to keep Mrs Alex360 happy, then strip the sports exhaust off my spare donor and the heater matrix, before finally getting to work on mine again.
Hope the ideas above help you and other builders, and I will post updates as I go.
Cheers
Alex360