The outside width dimension you are asking for isn't going to help you much since the machined features of the all uppers do not reference from the outer dimensions of the forging in the magwell area. The right side web is going to be thinner than the left due to the cut for the dust cover.
Per ARDEC Drawing 9349063 for an A2 upper, the width of the [b]inside[/b] of the magwell opening at the lower edge of the upper receiver is 0.925 +/- 0.010. This is probably going to be your critical dimension. I know of no reason this dimension would be different for an A3 (flat top) upper.
If you are looking for a flat top upper with lots of excess material thickness in the magwell area that can be opened-up, then you might want to consider the DPMS High or Low- Rise uppers. They are a rather thick-walled aluminum extrusion with no dust cover or associated cuts.