Moldman, I think I've talked to you before if I'm not mistaken. I too installed a 2500 pound remote control winch. It's a Badlands remote control winch from Harbor Freight. For the backing I used 1/4" plate steel. Can't remember the dimensions but make it as big as you can covering the upper body and lower tub as far down as possible. I had to put a slight bend in it to follow the contour of the lower tub. This plate is only drilled and bolted to the upper body as to not drill below the water line. Hope this helps.
What it lacks in ground clearance it makes up for with traction.