Sump Size, Depth.

[Hugh Mann]

Nice, that helps quite a bit. More so because according to that Triton method I should have a refugium of 25-50 gallons alone! Seems excessive to me, but I am definitely no expert.

At a personal inspection, the 80 is actually 17" wide and 13" deep. Seems like an oddball measurement to me, but the glass in intact, if dirty. Thinking it's a no go, as the pictures I saw originally didn't show the holes drilled into the side for plumbing.

 

[Hugh Mann]

Right.
So.
The space under the stand is just a touch too narrow to use a stock tank. Most vexing indeed. 75 gallons sell in my area for $400+ used. Might as well buy new, or a proper sump at those prices.
Not entirely sure where to go from here.