This sounds a lot like what happened to me recently, and it was my water pump. The pump only had about 2000 miles on it, so the bearing was fine and there was no free play in it. What happened was that the impeller separated from the shaft... see the picture here. Needless to say it wasn't moving any water around the cooling system.
It was a Stewart pump too, so I guess you never know when something freaky like this will happen. They did rebuild it for me so that's good.
The purpose of welding a distributor is to lock the advance to a set amount. This is very common for roundy-round racers, it is suppose to keep the delay in acceleration out of the corners from happening. Most of the time timing is set higher then normal engines would be for racing; this will create the engine to run hotter. From what I have seen 240-260 for a roundy-round racer is normal temps.
If you are using this motor for street driving then I would swap out the distributor due to the timing is locked in.
How slow is slowly rising to 240 degrees? 10 minutes, half hour or a hour?
It is hard to tell if the water pump is to high volume by watching but I have experienced this on a 85 Z28 I had. I had a high volume water pump and the car ran 240 but someone suggested going back to the stock pump and the temps dropped down to 210 after that.
I know what overheating means what I was looking for is what temp you thought overheating was.
Where is temp sender located?
240 is what a newer aluminum headed small block can run at. Is the radiator boiling over?? Do you have erratic temps? Does it stay stable at 240? Everybody goes by the old 180-190 but modern systems run hotter.
And what radiator cap do you have? Going by memory every 1lbs ads 10 degrees?? Of course staying within your systems limits. I am not a parts changer as that just gets you know real answers.
Brian Hobaugh SCCA National Tour June 2014
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