You state that the cooling was better with the new Radiator from Performance, is it the aluminum single row radiator they sell? I would also up grade the Fluid in the Blue fan clutch. Typically the water pump does not fail in the manor that you are thinking about, where it does not push fluid.
As for the lower radiator hose, if the hose is real old and soft, then I think they could suck it close, but for the most part, the hose is short and sturdy. I have not seen any 80 series stuff suck the lower hose to the point of closeing off.
But I do think you have a different issue. How long did you perform the Bubble test and at what RPM?