AFAIK, the difference between the 31 gallon and the 40 gallon is the depth. I have a 40 gallon my Suburban and the width and length are perfect. And since I have 4WD the depth is no problem. It looks like it came that way from the factory.
To do so, you need to remove the rear crossmember and move it toward the rear. I purchased another (same) crossmember off the parts board and installed it ahead of the tank. This holds the tank at the top and allows for use of factory R-body tank straps.
Not mine but you get the idea:
Like lowrollin said, this puts the filler neck in the wrong position. If you can find flexible enough filler hose you might be able to heat the filler tube on the tank and swivel it around to make it work. I relocated mine to the corner also.
And since I went fuel injection I bought a fuel injection tank. I don't know if a carb or diesel tank are different, but the filler neck is about a 1/4" larger diameter. I had to glue a filler piece in the filler hose to mate to my filler tube.
Another view of mine:
