Some details:
I got the rear axle, exhaust, and suspension all out of the way and stuck the 9" under there to see how it fits in relation to the gas tank and wheel-wells.
I can move the axle a little bit back from stock without moving the gas tank:
I want to get it to close to 110" wheelbase, and I want to maximize departure angle, so I started looking at how I could easily move the axle back a few more inches. I don't want to move the tank up through the floor at this point, but it looks like I can move the tank back 3-4 inches easily.
So I took the tank off and found there is a little "saddle" under the floor that the tank rests in. In order to move it back I will have to cut off the back part of this saddle and weld back in a filler piece that doesn't drop down. That's a bad description, so it's a good thing I took a pic:
Next time I get a chance to work on it I'll cut down the back of that saddle and rework the tank so it mounts all the way back.