You need to know the true temp. of any motor before throwing parts at it. If your hitting 260 deg. for any period of time your in the boiling or danger zone and damage can be done. If you truly did hit 300 degrees I'm suprised your not sweeping up pieces of motor off the highway!!

I agree with my7t1, if the temp reading is true,I think you have a cracked block. the oil is one of the first things you should of checked

If the top hose was cold and the engine was hot, you definitely have a circulation problem. Exactly the same symptoms I had on the '86. Did you try filling it through the heater hose and letting the coolant run out the radiator top? That insures you have the system full with no air pockets. If it's still hot with a cold upper hose, sounds like your water pump has laid down.

I think I remember that it didn't have a t-stat in it when the problem cropped up, so that eliminates a defective t-stat. Just for the helluvit, you might put the stat in a pan of water on the stove top and watch to see if it opens before the water boils......

Dave

Stephane, fill the rad. to the top(within 1-2"), put the cap on, and make sure the overflow is full. Then run it long enough to get to operating temp. It may take 20 min., but it really sounds like its just not full of coolant.

When it gets to op. temp., shut it off, and let it sit for a while. That will allow the engine to cool enough to draw the coolant in the overflow into the rad. Watch and make sure the overflow doesn't get empty. With the engine hot, once it cools some, it will create a vaccuum inside, and thats what draws the coolant from the overflow back into the rad.
You will notice, while its running, that the upper hose will collapse, and then open back up. This is when the vac. is present in the system, and drawing the coolant into the rad. If you run it without the cap on, once it starts getting hot, the level in the rad. is going to go up & down, and you're liable to get a face-full of hot coolant. NOT FUN!! Fill it, cap it, run it.

The temp sending unit should be in one of the heads, between the two front spark plugs on the drivers side, or the rear two plugs on the pass. side.Adams' Apple38660.8451851852

Oil pressure usually tends to drop off as the engine heats, the more wear on the motor, the more noticeable the drop. I'm wondering if you might be having some kind of elec grounding issue along with the heating.

[QUOTE=daveb12] Oil pressure usually tends to drop off as the engine heats, the more wear on the motor, the more noticeable the drop. I'm wondering if you might be having some kind of elec grounding issue along with the heating.[/QUOTE]

Me too!
Also, if it really was running that hot(according to the gauge), and it didn't puke it's guts out when you shut it off, I would question the validity of the gauge. Adams' Apple38662.7594328704

Don't overlook the lower radiator hose. It has a spring in it to keep the hose from collapsing at highway speeds from vacuum pressure. If the spring has deteriorated and rusted away, the hose could be collapsing keeping coolant from leaving the block to the radiator to be cooled. When you slow down to lower speeds the hose opens back to normal shape and allow coolant to pass and the engine temp declines.

Your drive around town and then the 90 mph run seems to exhibit those symptoms.