Tom
If I remember correctly the backspace on the original BRM wheels was 4 7/8", so with an original track of 56", that would leave 9 1/8" on each wheel. So 56" + 18 1/4 makes it approximately 74 1/4". I think that pin drive adapters might add a total of 3/4" to the overall width on some setups. Overall as long as suspension and body work clears I think most chassis will end up in this general area. I have no adapters on my rear uprights, but my bodywork measures 80" inside lip to lip, and I wanted to fill the wheel opening without rubbing.
I will take some accurate measurements and see exactly what my track is.
As you said let everyone decide for themselves, whatever works.
Cheers
Phil