I am doing the exact same thing, but am drilling the sump. I know about MAG's leaking. I have run one for 4 years on my computer. Yes, computer. It's water cooled
. Best advice, pull the 4 screws that hold the cap on, spread a thin layer of silicone on the O ring and re assemble. Then, apply a nice bead of silicone around the seam. Mag's are cheap enough that if it leaks in a couple years, or gets full of slime and gunk in the rotor, just buy a new one
Also, mags will suck, after tube is primes, but will be a pain, and they loose efficiency when they have to suck water