What could be wrong if a stock 78 Camaro Z28 350 overheats only over 55 mph?

The most probable answer is: Coolant is not flowing through your radiator. Coolant is low/empty, radiator is smashed or clogged, but most likely the thermostat was not installed correctly or has failed. BUT - if any of these were true you would also overheat in stop/go, 30 mph traffic. If you can drive all day at 30mph stop/go and not overheat, look at the next answer.

Another answer: The automatic transmission is slipping in 3rd, creating a lot of heat at highway speeds. If you can see gray or sooty tranny fluid on the dip stick, keep driving it until your clutches burn and get a new 4 speed (200 4R or 700 R4). Don't rebuild the stock 3 spd.

Just FYI, at 55 mph the fan isn't doing anything anyway. And I can't believe it's really "only" at 55 mph. For example, if it started over heating at 55 mph and then you slowed to 25 mph stop and go driving, it would cool down?

fan shroud not installed,thermostat in backWords,air flow blocked.

If your car is an automatic like my 78, the only definite solution I know to this common problem is to trade in your transmission for one with overdrive. You can notice that the car won't overheat at the low rpm's you encounter on most surface streets and then when you get on the freeway, you hit 4000 rpm and almost blow up the engine in the slow lane. Personally, I'm going to take out that automatic and put in a five speed manual. Ultimately the investment should pay for itself with the fuel economy you'll probably get, not to mention that a three speed automatic tranny will almost definitely just burn itself to dust in the modern world anyway.

i had lot of different years if you replace the clutch fan with a streight fan it will over heat at a high rpm due to the blades streighting out

I had a 86 TA, the air dam would drag and came off the car, I didnt think anything of it so at 55 or higher the car would over heat, make sure the air dam is attached that runs along the bottom of the car. air dam under car missing is 100% ypur problem.at higway speeds air goes around radiator,need lower spoiler to divert air to radiator g