did you accidentally double gasket the filter? ie slap the new filter on with the old filter seal still in place?
the sending unit has no effect on oil pressure-- it's only a "switch" that reads the pressure and transmits to your gauge. leaking 5 quarts from there would be quite a feat.
i'd suspect else where... either like at the filter like i suggested above or possibly you lost a galley plug out the back of the block.
what's the connection at the block for the filter? 90* or the flat plate?