If the cooling passages of your classic are clogged with silt, it can lead to hot spots in the block and potential catastrophic failure. If you don’t want to remove the engine from the car and strip it completely, it’s often effective to fill the engine with quite a strong solution of kettle or dishwasher descaler, run it up to temperature then let it sit for a week before flushing with a high-pressure hose. Of course, this may have a limited effect on passages that are completely blocked. The descaler can only nibble at the ends of the blockage, rather than softening it along its whole length.
It’s probably a good idea to remove the cylinder head and undertake as much manual cleaning as possible, both of the water jacket around the cylinders and the various passages in and around the cylinder head and water pump. Use a high-pressure air or water jet. Alternatively, obtain a wet-and-dry vacuum cleaner and reduce the suction nozzle down to a length of flattened copper pipe, which will scrape and suck at the same time when used to probe the depths of the block.