I am going to R&R my head to add a higher compression unit. So today I disconnected the radiator hoses. Plugged the inlet hose on the side of the block then poured in a gallon of Muriatic acid. I let it sit for a few hours then removed the plug and let it drain. Stuck the hose in the outlet on the head and flushed out huge chunks of rust and grime. A flashlight in the block shows clean bare metal. Tomorrow I will switch the head and fill it with coolant and anti rust. Hopefully it will run cooler.
I guess it would be a good time to back flush the radiator too.