Glenn I've had my '68 427 for close to 25 years and I've been through all the overheating issues and solutions. Firstly the fan cowling doesn't fit tight over the fan blades - it doesn't really need to.
Lots of people bang-on about electric fans (usually twin units mounted in a frame and clipped to the rear side of the radiator. These assemblies is fine for stock small blocks, questionable for a big block. You have two relatively small circular fans pressed-up against the rear of the radiator. When operating they will only draw cooling air through part of the radiator (two small circular areas). You'll possibly need to upgrade the electrics to cope with the current draw from the fans.
My advice would be to check that the viscous fan coupling is working correctly - they do fail.
There are various rubber and foam seals surrouding the radiator and shroud where it fits to the forward bulkhead and under the hood - they need to be in place.
Later C3's had modifications to the front end to improve airflow and cooling within the nose cone.
Aim for ensuring that ALL cooling air coming through the nose cone ONLY passes through the radiator and doesn't 'leak-off' elsewhere. The plenum area inbetween the fan and the radiator surrounded by the shroud is essential for best cooling.
Has the engine block ever been chemically flushed-out at any time? - is there a build-up of crud in the galleries? - has it always had a 50/50 mix of antifreeze.
Is the radiator in good order (without a layer of crud on the bottom)
Is the water pump the original unit? - if so it may have had a metal impellor? - the vanes can get eroded over time from dameage by cavitation and lack of insufficent coolant - check it out perhaps?
Is timing correct? - if running too retarded it can cause the engine to run hot.
Best radiator and generally recommended by most Vette owners in the States is a DeWitts alluminium OEM style replacement. They are are expensive, very expensive. But they'll last the like of the car if run with 50/50 coolant.
I've had mine for perhaps 15 years or more and its efficient enough to keep my 570 hp 489 cu in '68 generally within the 180 to 190 degree temperature band.
I've never had a boil-over. In fact during winter runs at constant 50/60 mph, other than in town use it struggles to reach theses temperatures. Summer use is generally fine but can get hot if caught in traffic jams, especially on humid days and may reach 200 degrees.
Thing is - after 60 or so years much of the OEM applied heat and sound insulation has deteriorated and irrespective of whether the engine is running at an ideal temperature.....and you will 'cook' inside the car - its not neccesarily overheating - just running hot (as its supposed to). There is a huge lump of metal up front emitting loads of heat. I have found that thermostat selection seems to be quite important.
Best of luck.
