I just got a call from Scott Stastny of Deluxe Autowerks in West
Chicago Ill.
He had been working on a 1953 Hardtop with a 1963 drive train that
constantly overheated when idling.   Scott is the Stude expert in the
Chicago area and is the best mechanic I have ever met.
He used modeling clay and found that there was .128" clearance between
the impeller and housing.
He scrounged up a N.O.S. pump and found that that pump only had .020 "
clearance between the housing and impeller
He took a good look at the waterpump on the car and noted that the
impeller was much smaller than the N.O.S. pump and of a different
design with smaller impeller blades.
The repops are supplied with a much thicker gasket and that adds to the
clearance problems.

The pump in the car was not pumping enough water through the engine.
This is especially bad at idle.
These are repops sold by various vendors.
The casting number on the repop was 527070 which is the same as the
N.O.S. pump but the letters are 5/15" high and on the N.O.S. pump the
letters of the casting number were 1/4" high.
Service letter S -1048 dated July 10 1962 states that the distance
between the mounting flange and the outer tip of the impeller should be
.938" but on the repop  that distance was only .834.
The fastest way to recognize the repop is that the hub where the
fanblade bolts to is .280 thick and fully machined.
The N.O.S. pump has a .230" hubflange and is sheared and rough on the
edge.
He found an other repop that has the exact same dimensions as the
N.O.S. pump and is of identical design.
There may be a bad batch of pumps out there.
Installing the N.O.S. pump solved the overheating problem on the '53.
I noticed a discusion on overheating in "Turning Wheels" and this may
help.

Electric fan are always more efficient pulling rather that pushing.
mount it up close to the radiator.