[Christ]

There are cases where blocking grille openings actually will increase airflow through the radiator, though I can't think of any right now.

Putting a gasket between the hood and upper rad brace is a well known way to improve cooling, as well as a tray under and around the radiator. Most of the air that goes into your front end doesn't really go to cooling your engine.

If you want to compare apples to apples, leave NASCAR out of the street car arena. NASCAR autos have tuned inlet ports on the front of the car that allow just the right amount of cooling based on the size of the opening without sacrificing aerodynamics the way street cars do.

Blocking your grille properly could allow air to flow through the radiator uninterrupted by cross-flows under the hood, which would improve cooling over stock. This is part of the reason that newer cars have radiator trays above and below, and a large part of the reason for adding fan shrouds to radiators.

A full belly pan also does not necessarily result in a hotter engine, as long as your cooling system is adequate and you have adequate airflow to vent the system into open air.

Do you have evidence to back up what you're saying about a hotter engine having reduced efficiency and less life? Seems funny, because many engines run overcooled OEM style, and end up getting better efficiency once they're allowed to run at some nominally higher temp.