The Eaton SCs of this era have some diagonal slots parallel to the main output port that some claim reduce the efficiency of the SC.

There are lots of opinions of the function of these slots, and the benefit of blocking them on various make/model forums, and various approaches that some have used to "seal" them such as welding in, epoxy (bad idea, falls out), and blocking plates.

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The slots are for noise only. Blocking them has no effect. But man will that make your car loud!
The Ford Lightning guys have been doing that for years. Sounds like a jet aircraft!

See this link. One of many.
Blower Noise

People have blocked them for a long time and many claims have been made. No facts have ever been presented.
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"The two oval slots on the output plate are air leakage ports. Their only purpose is to decrease operational noise from the blower. Arnie, from Eaton Corp., explains their function like this, "When you stomp on the gas, the boost shoots up and the air flow goes supersonic. The air then slams back into the output plate and through the leakage slots, which reduces the hammering of the air into the housing causing less noise. The OEM can not have such loud NVH, that’s why we put them in the housing. The 2 small ports are not there to feed air, only to reduce noise." Basically, for the best flow and most efficiency, these ports can be blocked off, making the outlet port (triangle slot) the only port on the outlet plate of the housing. Closing these ports will not increase IAT's; however, it will make the supercharger much louder when in boost. As an example, aftermarket supercharger companies, such as Harrop, Roush and Magnuson do not use these slots in their blowers."

Best way to plug them is with QuikSteel.