Need rear wheel backspacing suggestions for 65 valiant with mini tubs and inboard spring kit

My axle is 55 wide. Wheel mount to wheel mount. I run 15x10 6.5 back space.

View attachment 1716362933

View attachment 1716362934
Thats an epic fitment, obviously tubbed but did you also have to do work to the outer side of the tub as i am finding that to tuck like you have my rubber will hit the outer side of the tub as it steps in from the quarter panel.
However i am only using a 275 on a 8.5inch rim with shortened diff and spring offsets