I make my own "high flow" pumps, much of the difference is in the impeller design. If you look closely at your old pump and did a dry install with some plumbing putty stuck to a impeller blade you might find that the impeller is more than 0.050" away from the casting in the block. The reason why they do this is so if the bearings fail the impeller blades won't strike the block casting. Only problem is you need close clearances for the pump to move water efficiently, I always move the impeller out on the shaft to get 0.005"-0.010" clearance at the impeller. This makes a huge difference to the volume and pressure the pump is capable of.
As for how to make you own "high volume" pump you will need to find an old impeller that has cast vanes with the curvature to the impeller, most of the ones I see today are the sheetmetal straight vane type. These are inferior in every respect but cost. Set them close to the block and voila, "high flow" pump.
I know I know, the aftermarket ones have larger passages etc.etc.etc. but honestly if you set the pump up right with nice tight clearances they will perform about the same. It's getting harder to find these old impellers now, I grab every one I find in the junkyard. Keep your eyes peeled, they are out there.
As for the impeller hitting the casting...if your letting water pumps go to the point where bearings are failing...you need to find a different hobby.
They really aren't necessary for the street anyway Kaylah unless your running a supercharger or a turbo. Save your money for something that will make a difference in performance.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo