The drain is 3/4"
I'm lost here somewhat. What year is your hull that there is not a flat portion at the keel
Are your mounting a bracket on a 20'? If so and Don is doing the work, I'm positive he can and will make it work.
I would think that once the drain sleeve is mounted for measurements in the center, it could be fared out all around without difficulty, then screwed in with 5200.
Here is a close up picture of the drain.
