The key is to ensure the plates within the timing cover are sealed properly and not corroded as well the grommet sleeves.

Secondly, Cast impeller waterpump. NOT stamped metal.

Third, correct core size for engine. Core is not corroded with crap.. Ensure cooling system properly purged (burped) so there is no air in system.

New proper pressure radiator cap.

Proper pulley size

Fan shroud proper fitment on core support and proper spacing within shroud...and sealed all around the core support

Make sure timing is correct and carb is not lean.

Don't waste your time with "water wetters" "snake oil" cures or an electric pusher fan until you alleviate and correct all the common problems first. These cars ran fine 40 years ago and age has deteriorated main cooling components so expect an increase in temps if you have a 30-40% degredation in corrosion, clogs and air in system. If the engine has not been rebuilt, the block and heads will have quite a bit of sediment and corrosion floating around and blocking cooling passages. I rebuilt a Pontiac last year and got about 3-4# of crap from within the block after baking, shotblasting and shaking.