With a wider wheel wouldn't you nee more BS in order to have clearance on the outside?
Yes, that is true, but that extra clearance on the outside comes at the expense of clearance on the inside. You have the wheel opening lip on the outside and the leaf spring on the inside, and with E bodies, those leafs springs are not parallel to the frame.
I'm not positive what the spacing from the face of the brake drum to the rear segment of the leaf spring is. This measurement will determine how far back your wheel/tire combo can go.
With the wheel tire combo in my avatar, 4.75 back spacing was the max. However, with a 295 width tire on an 8" rim, it has substantial sidewall bulge that a 275 on a 9.5 inch rim won't have. I think the sidewall bulge on mine was around 2.5 inches, whereas the combo your looking at may be around .75 inches. That's a big difference that could accomodate a considerably different wheel back space.