Back spacing question

[djw32]

Guys,

I'm trying to figure out my wheel dilemma.

I would like to have all wheels and tires the same on my 70 Chevy (so I can rotate them). I plan on running 255/40/20 or 275/40/20.

Here is what I want to run:
2"(cpp forged) or 2 1/2" drop spindle(Mcgaughtys)
captainfab's camaro rear brake brakets

Is anyone running something similar to this?

Thanks for the help.

David