Good question on how to measure with everything mounted.
Although there are a few differences which I think could all mess with backspacing.
I already have discbrake ('73 A-body) spindles on the car, and the Wilwood setup I'm looking at seems to mount and position the adapter (to which the rotor mounts to) 'behind' the hub.
On your setup the adapter, I think is called a 'hat' and mounts 'over' the hub from the front. This alone would increase the trackwidth by the thickness of the hat.

I'll shoot an email to Wilwood to see if they provide any spacing info compared to a stock setup.