The New 50  Gallon Holding tank has arrived and it fits.  Folk at MAS Precision Plastics did a great job. (out of Toronto) 
We added an angle to the box that allows you to slide it in the limited opening.  All the fittings are accessible through the closet floor behind the stairs.

Under the tank, the old plywood was replaced with Bullet board by the previous owner, then I cut it out to replace tanks, and then put it back in.  Strong stuff!  When I stand on it, there is NO bend at all.  (I'm 225 lbs.)

I realize the pictures make it look like there is no room to get pipes through, but there is plenty of space.
I have the CAD drawling if anybody is interested let me know.