[Graham]

Your description of this proxy says 

Production based Saloons/Sedans which entered Car racing between 1960 and to date

Does it matter that there's crossover?  For example, the Ford Anglia 105E and Mini first saw competition in 1959

I was meaning that perhaps these MR Slotman cars would be better used in another proxy when we have a wider range of cars available, and that probably would be a 50’s period. 
Making a special provision for just one brand car or perhaps two in the next month, is not really justifiable. 
The actual cars could still be entered from other brand kits etc.