I hatched an idea today MFK...
Since the stock tanks walls are slightly angled, the bulkhead and plumbing would be slightly on an angle as well, meaning my pump wouldn't sit straight. HOWEVER, what if from the stock tank i had a short piece of pvc, then a barb fitting and clamped some pvc tubing to my pump a foot away which will be screwed down. The tubing would provide some flexibility.
I can make a lid for the stock tank out of 3/4" styro insulation, and put one of those 3 drawer storage units full of bio balls in it.
Aw yeah I think this is going to work.
So it would be water from tank into the top of the tower, down through filter pads and bio balls, then whatever else I want, out of the sump through a little tubing to the pump then into a manifold which all 4 of my returns will stem off.