I had thought about plumbing the two cooling systems together, but that would mean running larger hoses front to rear. Something I want to avoid. Under the car they would get wiped of. Inside the car would be too intrusive in what is a standard mini interior, plus the heat. Plus the engines will be more dependant on each other to get along.
For now I will install a new two core radiator at the rear, instead of the second hand Leyland one that is there now. This will retain the standard cooling setup in the rear blowing hot air out the rear guard. I also have an electric fan in the rear (RHR guard), blowing fresh air across the motor.
If it does get hot with 1275’s, I will put the clubman heater core in the rear heater circuit. Mount it up the front with copper pipes running along the tunnel on the inside. Carpet should cover the pipes up. I will also run the standard heater in the rear circuit.

sounds like you've got it all nutted out then - I went over your car at the Mill last year - just beautiful

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did I tell you that I won a trophy?