pellet-type cats

on pellet-style gm cats (this one is 1982), the pellets should be clearly visible with the refill plug removed, right? i pulled my plug, because i couldn't hear any rattle when i banged on it. sure enough, i saw nothing but the waffle-looking baffles, top and bottom. i failed emission inspection for high CO. i know it's probably because it's running rich (but very nicely). but would sticking a new monolithic 3-way on there be enough to get by? my CO numbers were about twice what they were supposed to be, and all other numbers were great.