...I always tell My customers..."If You find the Original Plate Installed in the Block, Leave IT THERE! If you pry it out you will damage the 'Press-Fit'. NO NEED TO PULL The Original Plate.
The ID in the Original Plate is LARGER than My Repro Plate, at aprox. .740" Plus .005" Minus Zero. The original 'Hat' as You call it, is at .640", by the Last One I Measured. Being that skirt is smaller, it was NOT Restricting much of a Flow.
Read this:...In Closing Off 95%-99% of the By-Pass Flow, Even More Coolant Is Diverted to the Radiator. This is a GREAT Improvement in Cooling Efficiency.
My Repro Plates And the SKIRTS are Machined to a Target of .723", with a .001" Clearence, that is they are Matched, Exactly. Yes, there still is a Flow past the Skirt so the Thermostat may Acclimate/Monitor to/the Coolant just returning from the Radiator. Openings: 180F=.260" 195F=.130" Know that there is a Vortex with-in a Gradiant.
MJ