Traction Issue

I just had my local machine shop install a new 512 440 Source stroker kit into my '70 440 block. The car is a 72 Challenger w/727, 3000 stall converter & Strange 60 rear w/3.23. I put the largest tires on the back that clear the fenders (255/50R17 Nitto).

The issue is when I go around 30 to 40 and nail the gas the car often pulls quickly to the right so I can't stay on the gas and keep control. I had replaced the shocks & leaf springs earlier and all is new. The car brakes straight so don't think the right brake is dragging to cause this.

Any thoughts?