You don't need to plug those holes you already drilled, consider them pressure balancing holes. Drill countersunk clearance holes for 10-24 on a 6.822 (Berkeley) or 6.913 (Dominator) bolt circle by no deeper than .625. It won't really hurt anything if you break through.
Use anti-seize on SS bolts.
If you bolt through into the oil reservoir, you will need to seal the threads. I would use Loctite. Again, I recommend against doing it this way.
It's going to hit the impeller nut. I put a 2.250 hole through the center.
