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Speaking as a retired chemist go with the oxalic acid (OA)approach mentioned above. Du Pont use to have a radiator/engine flush kit that used OA and sodium bicarbonate as a neutralizer.

Both muriatic and sulfuric acid are way to strong and aggressive for this application. Also, more hazardous to use.

I've used OA on about 5 engines with great success. While you cannot get the Du Pont kit any longer you can find OA available. Read the various post on this web site and you can see the best way to use it for cleaning your system.

My local boating supply store had Oxalic acid for sale on the shelf--I think it was intended for bleaching teak.

You can also get it at Walmart
http://www.walmart.com/ip/15026756?wmlspartner=wlpa&selectedSellerId=0&adid=22222222228009484846&wmlspartner=wmtlabs&wl0=&wl1=g&wl2=m&wl3=42978197912&wl4=pla-75604554529&wl5=1026201&wl6=&wl7=&wl8=&wl9=pla&wl10=8175035&wl11=online&wl12=15026756&wl13=&veh=sem

This is what I used and it worked fine. After I put it in the engine I flushed it out with water and air.

Yesterday a fellow suggested draining the radiator, block and head and refilling with vinegar. Circulate, then rinse with baking soda and water. Then flush and refill. What are pros and cons of Vinegar vs Oxalic Acid?

Oxalic is more effective than acetic (vinegar) acid, and less likely to cause problems; acetic will be aggressive to the brass and solder in the radiator.

You do not want to use strong acids: hydrochloric(muriatic), sulfuric, or nitric in your engine block. They do not discriminate between deposits and good metal and evolve nascent hydrogen which can affect the cast iron leading to brittleness/cracks. There's always one guy that thinks he got away with it and then tells the next guy, and the unsuspecting next guy has issues.
Depends on the source of the fouling, no one size fits all here.
If hard water had been used in the cooling system it will have a lot of calcium and magnesium oxides/carbonates in the deposits and oxalic will not remove it. Oxalic will form a hard insoluble solid with it(make worse). It's also the main component of kidney stones. Reaction much like soap scum(fatty acids) where the soap sodium fatty acid salt exchanges with the calcium to make the hard/insoluble salt(scum). CLR/limeaway type products use hydroxyacetic acid, different than vinegar(acetic). Vinegar is weak and just isn't that effective.
If it's crusty rust, then maybe one of the organic rust removers might be an option. Evaporust is a common brand and does not harm the other metals. You'd have to block off the outlets and fill the cooling jacket. Try on a small part first like the outlet/thermostat housing. You'll have to flush out the black ppt after cleaning with another flush. The problem will be you won't know the condition of the internal passages. Some may be rusted very thin and not hold up and your block could be trash.
They use TSPP in the Prestone hot radiator flush product. I've used it but sure it will not cure this issue. Takes away the oily glycolic acid residues mostly.
There are commercial passivated acids that are used specifically for cleaning cooling systems, but I'm not advocating that for the consumer/DIY trade. If you go to rebuild, they'll bake, wash, or boil out the block to remove the fouling and check/clean the passages.
The other thing is where is the fouling? In the block or the radiator, or both. You'll have to cure both to solve an over temperature problem.
This is one of the more common problems in vintage motors, and is the result of lack of care since new and normal aging. One can't restore everything in any condition. Sometime you have to change/upgrade parts. Consult with a radiator shop if the radiator is plugged.

You do not want to use strong acids: hydrochloric(muriatic), sulfuric, or nitric in your engine block. They do not discriminate between deposits and good metal and evolve nascent hydrogen which can affect the cast iron leading to brittleness/cracks. There's always one guy that thinks he got away with it and then tells the next guy, and the unsuspecting next guy has issues.
Depends on the source of the fouling, no one size fits all here.
If hard water had been used in the cooling system it will have a lot of calcium and magnesium oxides/carbonates in the deposits and oxalic will not remove it. Oxalic will form a hard insoluble solid with it(make worse). It's also the main component of kidney stones. Reaction much like soap scum(fatty acids) where the soap sodium fatty acid salt exchanges with the calcium to make the hard/insoluble salt(scum). CLR/limeaway type products use hydroxyacetic acid, different than vinegar(acetic). Vinegar is weak and just isn't that effective.
If it's crusty rust, then maybe one of the organic rust removers might be an option. Evaporust is a common brand and does not harm the other metals. You'd have to block off the outlets and fill the cooling jacket. Try on a small part first like the outlet/thermostat housing. You'll have to flush out the black ppt after cleaning with another flush. The problem will be you won't know the condition of the internal passages. Some may be rusted very thin and not hold up and your block could be trash.
They use TSPP in the Prestone hot radiator flush product. I've used it but sure it will not cure this issue. Takes away the oily glycolic acid residues mostly.
There are commercial passivated acids that are used specifically for cleaning cooling systems, but I'm not advocating that for the consumer/DIY trade. If you go to rebuild, they'll bake, wash, or boil out the block to remove the fouling and check/clean the passages.
The other thing is where is the fouling? In the block or the radiator, or both. You'll have to cure both to solve an over temperature problem.
This is one of the more common problems in vintage motors, and is the result of lack of care since new and normal aging. One can't restore everything in any condition. Sometime you have to change/upgrade parts. Consult with a radiator shop if the radiator is plugged.

Don't know why this posted twice. I tried to erase one but it's too late.
I did some checking around and Prestone Heavy Duty cooling system cleaner lists it as containing hydrochloric acid. However, it's a proprietary formulation so does not say if it's inhibited, sequestered, or offered as a hydrochloric salt. I'd still recommend to stay away from strong acids for the reasons I listed.

On a flathead Chrysler six I took out the core plugs and found the water jackets had 2 or 3 inches of deposits, mainly mud and rust. It washed out easily with a pressure washer. It was possible to do a thorough job by pointing the pressure in different directions.

After doing this you could fill with fresh water and add some CLR calcium lime and rust remover. Run for 50 miles or so and flush the system with a garden hose in the rad.

This should get the system clean without using aggressive chemicals that might cause damage. You may cause leaks by removing deposits that are plugging a weak spot.