Generally speaking it should be about even with the top of the radiator. Take a look at your modern vehicles. Sometimes they are a little lower than the top of the radiator, sometimes they're a little above it.
Since it isn't gravity that's controlling the transfer of coolant back and forth, it doesn't make too much difference.
A previous owner had already mounted it in the position its in and I decided to leave it in place. Unfortunately for him he had the an incorrect style radiator cap installed which would not allow the fluid to return to the radiator after it was expelled.
Be sure your radiator cap has the poppet valve ( Nickle sized rubber valve in the center of the cap).
In these pictures, I have just driven 10 miles this morning and the engine is at about 180 to 185° and you can see that the coolant level is about halfway up in the container. In the previous pictures I posted it is at the cold level.
1950 Chevrolet 3100 (Ol' Roy)
1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)
