You need to drain the block too. On the drivers side back near the bell housing you'll see the petcock or where the petcock should be (if someone removed it and plugged it).
If when you open the petcock and it just dribbles out, remove the petcock and it should gush out. IF not, probe into the block with a metal coat hanger and probe around as deep as it'll go until it does gush out. It should carry the debris out with it.
The water jacket around cyl's 5 & 6 being lowest tend to collect the most crud. Sometimes enough to where the coolant can't encircle the cyl's enough to carry heat away.
I'd recommend draining from the block 1st so theres enough fluid capacity to help clear the crud.
Good luck
Dave
