First, the water pump isn't going to like it. Secondly the screws won't be standard any more.

I'd first try something like WD 40 - Just in the holes (I don't know Corrosion X, maybe). Use the little miniture tube nozzle, keep it out of the impeller, etc.

Why were the screws "leaking before"?

What did you do over the winter, just antifreeze?

Couldn't you get things tight before?

The waterpump has a "safety feature" to protect the engine's linkate into the assembly that drives the shaft inside the engine: It leaks to warn you that the bearings, shaft, gaskets, etc. are failing by leaking a little raw water. In my experience that evidences itself outside of the screws (usually below the pump, just where it meets the engine block. Are you showing any evidence of leakage there, or is it just the plate of the impeller cover?