I can not remember your complete setup, so I might miss an important point, but here goes:
(1) B-Spindle setup provides camber gain which will increase front grip and decrease understeer (increase oversteer).
(2) Stock rear 4-link suspension at or near stock height, which has some toe-out which decrease understeer (increases oversteer).
(3) Stock front sway bar with ATR rear sway bar, which will
decrease understeer (increase oversteer).
(4) Stiffer springs front and rear which should be balanced regarding over/under steer if springs are matched set. If you have stiffer springs in back and not in front this will decrease understeer (increase oversteer).
If you have (extra wide) 275 rear tires this might be enough to offset all of the oversteer you have built into the setup, but I am concerned.
Try this, it will not cost you anything. Take off the rear sway bar to reduce oversteer. Because the swaybar increased the lower bar stiffness regarding side loading, I am not a real fan of removing the rear bar, but just for a trial it should provide good information.
If this reduces oversteer and makes the car more balanced, then increase the size of the front bar and re-install the rear bar.
Just an idea. Hope everything works out.
Good Hunting.
Donald McMullin