At a glance, it looks like you can lose a fair amount behind the rear wheels; look how long the area occupied by the gas tank is on the Lindberg kit. Yes to the above tips, do measure and get the wheelbase right first, but if you can lose some at the back, I would begin by cutting across the center of the gas tank on the Lindberg, removing a slice as wide as necessary, then, scribe both gas tanks free, patch the resulting trunk floor hole, and use the Johan tank glued in place to not only look more correct for the car, but to reinforce and hide a good deal of the splice seam. While it's cut free, you could take a bit of sheet plastic to give the gas tank a more accurate 3D appearance.