I hate to seem to disagree with anyone named Carl, but you might be able to do a fair job of flushing the system without such a compete tear-down. Keep in mind that dinosaur dung settles to the bottom of the block and even towards the back of the block - away from the radiator. You will come a long way by removing the rear pet cock down under the throttle linkage and steering shaft.
Most of those old engines will be so plugged up that nothing will run from the pet cock hole even with the spigot removed. So you need to go in with a piece of wire, like clothes hanger wire, and dig around in that pet cock hole until you get the primordial muck to run out. Keep the water running while you dig until the water runs clear. Then pull the freeze plugs and continue the process.
If after all this you decide that Carl with "C" was right, then every other part of the engine is probably in rough shape as well, and you might as well tear it down.
Have a great time!
Karl
