While idling, very little air is moving through the radiator - probably because your fan is in the wrong location relative to the radiator.

While driving, air is forced through the radiator regardless of whether you even have a fan installed or not.

Your best bet is to purchase one of pre-68 Dave's water pump adapter which centers the original water pump in front of the radiator where it was on the original engine.

Read about your issue on Dave's site below:

https://chev235guy.blogspot.com/search/label/Water%20Pump%20Relocation%20Adapter

Pre-68 Dave's email: [email protected]

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)