You are correct. You do need a higher front/lower rear to correct this problem. You may be able to slide the sights along the rib to make this happen. If not, here is something that may work.

Find out what velocity those slugs are, and shoot a faster slug. Under recoil, the muzzle jumps upward. The slug leaves the barrel while the muzzle is jumping. A slower slug leaves later (usually higher in the jump) than a faster one. So at close range, you can see a higher POI with a lower velocity load.

My Mossberg 500 with open sights shoots most 3" slug loads about 6" low at 50. Shoots dead on with 2 3/4 loads.

If this doesn't work, you do need to do something to change your sights. Try a higher shim for the front.