My just completed 74 360 Duster clone will idle at 190 all day long.Take it down the highway for 10-15 minutes at 55 mph and it creeps up to 220-230. New parts store replacement 26 inch radiator,factory fan shroud,factory aluminum 7 blade clutch fan,stock water pump/pulleys,and 185 thermostat.Any ideas?

Is there a spring in the lower radiator hose? If not it could be collapsing at speed.

Get the LM-1 on it Seth. The "tree" in your TQ may not be adjusted properly causing a lean condition. Check the A/F before you pull your hair out

I usually block the exhaust crossover in the intake (may not be your problem but it helps) you will then have to richen up the carb.

I can't remember if there is a spring in the lower hose or not,I'll have to check that.

The LM-1 is next on the list Tom,could be way off.

Just cruising along at 55 should not collapse the lower hose uNless he has a steep gear in it and the motor is really singing ...IMO.

I would LOOK at that situation ....try it ... but I am thinking the flow through the rad is somehow hindered. Many times I have seen and heard of the fan-action actually blocking air-flow at highway speeds.

Just pull the fan OFF when you take your next HWY-cruise ...and see what happens.

Is the radiator used and old? Could be partially clogged and not have enough cooling capacity. At speed the fan is rarely if ever the problem.

Change to a 8 blade water pump and put a cavitation disc on the back of the pump blades that should solve the problem. Even the 8 blade will help out esp at speed and lower the anti frez ratio to no more then 30%

3.55 gears turning 2700 rpm.Radiator is new as stated above.Fan is a factory piece.Distilled water with Redline Water Wetter.Did the same thing with anti-freeze too.

Two EZ things to check ... lower hose and then remove the fan.

Classic partly clogged/too small of rad symptom. Timing in the ballpark at what ever RPM your at at 55 MPH. Stat could b not opening all the way but less likely 2 b it

mine does this two, around town 180, hyw 200, everything is fine but i have a 6 blade WP that i put in about 10 years ago when i was racing it more often, now that its more of a cruiser i think i need a 8 blade.

Was this motor fully rebuilt? Just curious, as the small block is famous for collecting muck around the cylinder walls behind the freeze plugs over time. Had one that overheated like this a while back. Removed all of the freeze plugs on the both sides of the block, cleaned up the muck around the cylinders, replaced the freeze plugs and the motor dropped 20 - 30 degrees in running temperature.

...just got back from our local cruise.
..a friend had problems with overheating above 60 mph
..he changed the 7 blade fan for the 5 blade and problem was solved..
as stated it seemed that big 7 blade was blocking air flow at speed.