If we do it as a the El Piloto series we just spec it out… Even if we don’t do it as El Piloto the availability of parts will keep it fairly close to a spec series. I would at least spec the chassis and motor. The wheels and tires might wind up being a defacto spec, but folks can experiment.
The GT class has the most variation in bodies for a standard series, we could stay traditional for an El Piloto series with a Group C pack, or we could go with Porsche GT3 cars, or even something like a Lamborghini Murcielago, or Ferrari 599. Kinda mix things up a bit. 😉
I think that even if the cost to build one is break-even with the BRM’s we’ll realize savings in parts as the gears should last beyond one race. 
Build it strong.
Keep it simple.
Make it work.
(Leroy Grumman)