Firstly, a 1/8” gap is a little too narrow, by about 1/16” or slightly more. The hinge panel of the bonnet underpan is a fairly stiff 18 ga steel construction but it isn’t absolutely rigid, nor are the bonnet stops capable of rendering the bonnet absolutely immovable. Even when everything is to spec and the bonnet is locked down it can and will flex and move slightly while driving over bumps in the road and with a too narrow gap you risk chipping paint.
You don’t mention your vertical gap between the rear of the wings and the A-posts, which should be even and the same as the horizontal gap. Something is amiss if the gap at the rear of the bonnet is even across the bulkhead and down the A-posts but uneven along the sills. Not shims, or the bonnet subframe. This indicates that either the bonnet or the sills aren’t square, likely the former. Both should be perfect right angles. Easy enough to verify with a framing square. Not necessarily abnormal, and correctable by careful cutting and rewelding of the lower rolled edge of the rear wing quarter panel.
In my case, with the top of the bonnet perfectly level with the top of the bulkhead, the gap between the sills and lower edge of the wings was even but a mere 1/16” wide. This was due to the slightly sharper profile of the replacement MR outer sills. It required some surgery to correct.
You don’t say how much you had to shim the bonnet hinges to achieve the fit you have but if it’s really excessive (say, more than a half inch or so) and the car has been involved in a front-ender it is likely you have damage to the bonnet subframe and possibly to the hinge panel in the underpan. The problem with shimming the lower flanges of the bonnet subframe is the upper flanges will no longer be even with the picture frame and would require a tapered shim to achieve a solid fitment.