Longer spring shackles <span style="font-style: italic">would</span> raise the rear end -- however -- it would also bring the frame closer to the rear axle, which is suspended <span style="font-style: italic">over</span> the frame. TR3s in the stock condition have handling issues due to this arrangement, and raising the frame, relative to the axle, will make the situation worse.
Short of raising the whole body tub higher off the frame, I can't think of any solution that raises the rear fender away from the tire without reducing the frame-axle clearance.
If you really want those big tires, maybe you will need to flare your fenders?