Hey Big daddy,,,I had the same thing happen to me many years ago,,,trying to remember what it was.
Now that you have installed a shift kit,,,you have increased the fluid pressure in the valve body,( and your pan is pressurizing ),,off hand i'd say you have missed a check valve ball,,,or if your using a fluid re-director plate,,then the plate is not located properly.
Another thought comes to mind,,,,,,with the shift kit making higher pressure,,,you may have a worn seal, or a damaged seal or "o" ring allowing pump pressure to enter your tranny pan.