Grandville troubles
 

It runs hot in stop and go traffic. Then it will he hard to start. I flush the coolant 1st it doesn't look to good.

Did you use a two part flush?
That type is the best, but you should by pass the heater core for the flush.

Is there a fan clutch and does it keep the fan engaged at engine idle speed when the motor is up to temp,

Does one of the exh manifolds still have its heat riser flap and if so is it jammed closed?
Is the exh system bent up and restricted?
If the vacuum advance is not working that can contribute to low speed hot running.

You might just right off the bat pull the water pump to make sure the two seal incerts are there in the divider plate and are in good shape and also to check the water pump impeller to divider plate clearance.

Also just to look at if the pump has the preferred good cast iron impeller or if it’s crappy type that looks like an Alternator fan .

The motor was rebuilt before I got the car. It has a 455 with Kre dpors on it.

So If I can stop the fan at idle wouldn't you think the fan clutch is bad?

Sure would!

No, being able to stop the fan at idle would not necessarily mean the clutch is bad.

A frightening test from way back in the day was to use a piece of soft wood, like white pine. You were supposed to "gently apply" light pressure to the back of the fan blade with the engine idling on a hot engine. The fan blade was supposed to chip away pieces of the wood and continue turning. If the wood easily stopped the blade the clutch was shot. The good old days of analog testing! Pretty sure this test was from Gates, Dayco or possibly Hayden. Been many decades. My test is on a hot engine, 200F+. Gently raise the RPM's, the fan should turn close to engine speed and sound like a wind tunnel. Shut engine off and turn by hand. It should turn pretty tight.

Are you running vacuum advance? If so, is it connected to a ported or manifold source? What is your base timing?

Yes, It all depends upon how loose, is loose...:Ithink:

So i changed the fan clutch and let the car idle for 30 minutes. I then drove it for another 15 minutes. Now the temp gauge stays put no more up and down and you can feel the air flowing into the engine compartment.

Nice easy fix and good to hear.