I have a 95 and everytime i drive on the highway and put it in 6th gear...the temp gets right up to 240*
i just got a new radiator and hoses replaced ...they have been burped of all the air.... and it still goes right up to 240.
But when i drop it down to 5th the temp goes right down to 218*.
anyone have any suggestions?

Thanks

 

Checked your cooling fans? Does the a/c make it overheat? [8D]

 

the fans arent supposed to come on above 35mph regardless. 218 is exactly cool either, especially for going down the highway. you may have other problems like a clogged radiator, stuck thermostat,or something else

 

You should have enough airflow through the radiator at highway speeds so that isn't the issue. You mention the radiator is new.
Are the baffling and defectors under the front bumper missing or bent? These are critical at diverting the airflow though the radiator.
Did they replace the thermostat? 205-210 would be the maximum temperature that I would be comfortable with.
Any higher than that is a sign that something is not right. 218+ is a death sentence for your LT1. The oil temp is probably very high too.
Have a shop with the correct equipment do a pressure test on the cooling
system. It's possible you have a blown head gasket or an internal blockage.
It's also likely the water pump is not circulating the coolant sufficiently. That's where I'd look next.