It appears the rearend may still be set back a bit further than stock, which may be affecting your gas tank clearance. I know on my car the tires are closer to the front of the wheel opening, and yours seem perfectly centered.
I think you are right. WE checked on my other '69 Cuda with an 8-3/4" axle and there is more clearance to the tank. Will pull it forward a bit more and see how it looks.
