Technically, the only bodies legal for the BRM series are the BRM 962 and the Toyota 88C. The forthcoming Sauber Mercedes C9 will be eligible as well, although it will need to run a red motor to keep things even.
That said, I would not be averse to someone swapping a different body onto an otherwise completely stock BRM roller chassis providing it meets all the other criteria (dimensions, weights, etc...). In other words, no underweight body without an interior built to have an advantage over the stock BRM. This eliminates altering the wheelbase and/or guide lead length, etc... regardless of the wheelbase of the car chosen to model. The 956 fits well on the Toyota chassis but the wheelbase is off on the 962. The Jag XJR series go well on the 962 chassis with trimming of the venturi areas.
A word of caution with the model bodies though- with the exception of the Tamiya 962, generally speaking, all the others are significantly thinner, lighter, and less durable than the BRM bodies. Running them at Too Bad may be hazardous to their existence. It also means that ballast will likely be needed to attain min weight.
The intent of BRM is to provide gorgeous, scale appearing cars for racing. My intent with sticking as close as practically possible to the Challenge rules is to have box stock cars (with the exception of gears) and make this a driver's contest without continuously escalating tuning wars. If we stick within that objective, I'd let it ride.
But I'd also reserve my ability to change my mind. Anyone who picks up a box stock car should be able to win with the right combination of driving and luck. If body swapping and associated modifications impede that, it'd need to be controlled.
Clear as mud?
Chris