From your picture comparing the two tanks, it looked like the distance from the fuel pump mounting ring to the part of the LC tank where it slopes steeply for the suspension linkage pocket might be the same distance as it would be to the back of the GX tank.The tanks are indeed different shapes, but the only "non-fit" area was what I out lined, otherwise it looked like it was going to fit, just needed to be slid back a bit to get the tank straps to line up.
You mentioned that you had to slide the GX tank back a bit to get the straps to line up, could that be the cause of the interference?
If you line up the fuel pump mounting ring on the GX tank to the hole in the floor and move the strap location instead would it fit?