It just occurred to me that if you are successful in getting the water pump built to better specs, all my divider plates I have altered won't work with the new unit.
In a more serious vein (pun intended), setting a short impeller at the right depth will leave quite a void behind it. This probably isn't much better than too wide of gap in front - at least we can hammer in the plate and arrive at a proper clearance. Best solution is still a thicker impeller with deeper veins.