It seems you could take fixed arm loose, then drill a new hole in fixed arm, or elongate the existing holes to allow enough movement for better alignment.
I know the fixed arms on mine are sealed along the edges, but I think it could still be moved a little and resealed.
After looking at mine, it seems you'd be best off to remove the brackets, and replace with something similar, maybe even use an aluminum bracket. On mine the existing plastic brackets are not big enough to drill or slot, to gain 3/4 of an inch.
Jerry