I had great luck with the Evaporust treatment that someone posted here. Prior to Evaporust, every coolant change water would only dribble out of the drain even when I used wire and brushes in the drain. This year after I drained the block and let it dribble out for several days I used Evaporust. I took 3 treatments - I let it sit in the block for a day or two each time. After waiting each time, I removed the fixture on the block and used a shop vac to suck out the gunk. The third time, when running the shop vac there was a "pop" and the Evaporust poured strongly out of the drain - no more dribble. I then rinsed it with water 2 times and the water gushed out clean. Then I rinsed once with coolant. As a hint, I disconnected the lower radiator hose L fitting and pointed it upward (Instead of down). That way I could pour the Evaporust into the block without pouring it into the radiator.
