2nd Gen Mini Tub Backspacing?

[TransAm400]

I'm in the middle of putting the rear suspension together but I need to give the wheel builders specs on backspace now.

1979 TA stock width rear, DSE tubs and Speedtech Torque Arm. Will be running a 18x12 with a 335/30. Fronts are 18x9 with stock subframe. Surely somebody is already running this set up and knows the backspace measurements. Any help?