So, what I meant wasn't that the inner tub and frame rails were in the same place, it was that the inner tub and frame rail location is the same on all of the A-bodies 67-76. Ie, on all of the A-bodies it's about 2.25" to 2.5" from the inner tub to the frame rail depending on where you measure. Otherwise we wouldn't even have to mini-tub our cars, because the inner tub would already be even with the frame. :D
But, even if you have the offset spring hangers, the springs aren't perfectly even with the outside of the frame rail, and the springs are 2.5" wide, so, the springs would still be in the way. You might gain a little clearance because you need more clearance to the inner tub than the springs because of the way the suspension can move, but, not enough to run a significantly wider tire.