I've got a sb 416, new everything, re-cored, wider than stock, 3 core 26" radiator; running a factory shroud; new thermostatic fan clutch which I tested using the "3/4 tape test",- the fan seems to be turning fine. the fan blades are seting about 1 3/4 inch away from the radiator. there is a cleared, fins streightened, condensor and a new trans cooler in front of the radiator. new, if I remember correctly, 180# thermostat which I drilled two small holes to allow water flow. Theromostat is opening and closing. running a 50/50 mix with temp-cooler poured in. Engine is timed at about 35/36 degrees all in at about 3000 rpm.
The car has a factory temp gauge, which seems to be reasonably accurate. The problem is when I get the car crusing after about 5-6 miles the temp gauge is midway between hot and cold. Then creeps up a little more, just past midway...stays there then I get it home... idleing it begins to creep up toward hot...I shut it off. I can feel the water at a fast-boil when I feel the (hard) upper radiator hose. Header temps will vary between 400 and 1000+ (at #6, previously just after break-in #4 and #6 would be starting to glow! now they are down to 6-700)
Could there be a blocked water passage? The engine was nasty in a barn for a while when I got it. The machine shop, (who's attention to detail I seriously question) cleaned it and bored it as well as rebuilt the heads. The engine builder has no clue. I have had the car running about a year, during which time I have maybe put 40 miles on it due to hot header temps and now overheating worries. Any suggestions are welcome! Please help.

What is the temp that you are rising to? Coolant will always heat up and pressure rise after shut down do to heat soak the first few minutes with no circulation. So far I don't see a problem.
Thermostat can not control temperatures above the stat rating - just keep them from falling below the rating and to reach operating temperature slightly quicker.

Glowing headers are indicative of late timing.

Before you start worrying yourself into an ulcer, you need to get a good mechanical temperature gauge so you will know exactly what the temperature is.
Did your engine builder remove the drain plugs in the block when he cleaned it? I'm betting no.
They can be a real bear to remove but you will be surprised what comes out of the block after 30+ years.
Once you find out the temperature with a good gauge, be sure to post it here.

New brass freeze plugs were installed after it was cleaned...I just question whether steel brushes were run through it to clean and ensure no blockage. When I worked at a maching shop 25+ years ago in high school I always ran brushes through the water passages in the block and heads because sometimes they would be clogged with dirt/flash/bars-seal.
I tried to run a mechanical temp gauge...I could not get one to fit in the stock location (motor looks factory, sans headers)
The motor, idleing runs hot! It i don't shut it down..The motor will be toast.

You can lay a cooking thermometer on the cylinder head or the top of the radiator after driving it around to get a reasonable temp reading.

What do the spark plugs look like? Too lean might make it run hot too.

What type of waterpump do you have? Rubber seal on the radiator cap in good shape?

Spring in the lower radiator hose?

I would also suspect late ignition timing.

Any ethanol in the fuel?

When idleing it will move to HOT, I shut it down just befor it gets there. And this is ideling, after a 5-10 mile run, for maybe 5 minutes. properly rebuily motors, particularly sb's should not boil over....Move to and past HOT. I've had lot's of cars, rebuilt many motors (I should have built this one myself..but thought a professional would best serve me being it's a stroker...my mistake) anyway the motor is running hot.

I run BP 93, however I wouldn't be suprised that ethanol is in the fuel. The carb is a demon-sizzler built thermoquad, according to him built for a stout motor; maybe on the slightly rich side. radiator cap is new with a good rubber seal. the ignition is a new MP electronic distributor with a chrome box.

Modern summer blend fuels with ethanol burn very slowly.

A good test would be to set the idle timing to 25° btdc. Just let the engine idle and see if the idle operating temp drops.

Also mechanical advance all in by 3000 rpm is too slow.

A new style mallory / mp distributor is very adjustable . mechanical advance slots are eaisly adjustable with torx bit. as well as advance curves with lighter springs.

I would set advance curve as quick as possible

all in by 1600 to 1800 rpm.

the temp is OK out on the road moving but there's only a problem at idle?(at home it creeps up to Hot). Unless your initial timing is not advanced enough then I would guess classic not enough airflow or inadequate water pump.

Is it a stock pulley set up, anything underdriven or did you mix and match componants? Maybe it's not spinning as fast as it should. I would set timing to 34 degrees max (recurved so it's all in around 2500). Are you running a hotter plug? If so maybe it's acting more like a leaner mixture. Compression ratio and overbore? Friction cause heat... break it in maybe it's still just too tight

Dan,
Do you have a leakdown tester? If so, you can see if the head gaskets are sealing correctly. Another way to check for this is to use a chemical indicator fluid that changes color if combustion gases are entering the cooling system. A company called Lisle Tools makes an inexpensive combustion leak detector kit (P/N 75500).
Even if either of these tests aren't positive, at least you've ruled them out as possibilites.

are you running vac. adv. i have a 360 that has run hot for 5 years. i did everthing that evrybody sugested. i recently put in a MP distr. and it started running cooler. yeah i am a happy man. above somebody said to adv it to 25 at idle and see if it helps. that would be almost the same thing.

Sounds like there's something generating heat, or the cooling jacket's crudded up. The block should be airless shotted, it blasts the crud off the cores, and boucnes all thru it, so everything gets cleaned. What oil pump are you running, and what pressure does it maintain? The latest 422 I did, even at 9.8:1 and +.060 overbore, runs cold with a similar setup. Only hits 180 on long light throttle hills in 90-895° heat. Good efficient engines dont run hot. And I think it's more than timing or fuel.