"the coolant had so much rust and sediment "

I don't believe that just finding the drain plug is going to do what you need/want to do.

In the old days... car engines would need rebuilding every couple of 100k.... and proper rebuilding was understood to be Vatting the block before starting back up with replacing stuff... This is a full immersion caustic bath and then cleaning that off the block... all done WITH THE CORE PLUGS REMOVED.... ( also called freeze plugs ).... This would take care of the rust sediment in the coolant cavity until the next rebuild.

Many of us are driving cars which the engines have never been rebuilt (with twice the number of miles old gas engines used to go )...and even many of those rebuilt were done without stripping the block and Vatting it...

I think what you need to do is take out the core plugs... and get a wand which will alllow you to blast the recesses of the coolant cavity while using a wet vacuum to get it out fast enough to allow plenty of unobstructed blasting in the corners...

When you just open the drain plug the flow is not fast enough....and the opening is not big enough to get that sediment out....and you have no way to ' excite' the sediment in the area away from the drain plug.

It might even be helpful to make your own nozzle if you can... their is a design in one of " The Home Mechinist's Bedside Reader" would be a good design.