opening rear wheelwells for big tires

It's going to take more than a mini tub. The factory wheel housing is only about 31 inches front to rear at the seam in the middle. If you are moving the inner against the frame with a spacer in the center, you still won't have enough room for a tire that tall.
And since the factory style wheel housing mounts to the quarter panel at the wheel opening, if you start increasing the diameter of the wheel opening, you will have to figure out a way to reconnect the pieces to seal it all up.