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I'd give the block drain holes a careful poking with a piece of wire or a welding rod or similar. The most vulnerable part there would be the threads.

The drains are probably blocked with scale.

Since you have removed the heads water neck elbows to allow an escape path you could also blow compressed air in through the drains to help clear the crud.

A really good flush with the garden hose should help also.

If a motor full of scale has been left with a dry coolant jacket for any period of time the scale can go rock hard so it might pay you to fill the coolant jacket with water and allow it to sit until next weekend before tackling it again. This may allow the scale to soften.

The idea of putting a solution of CLR into the coolant jacket is attractive but I tend to wonder if this will attack the head gaskets and 'O' rings. Maybe someone else can give you better advice on the advisability of the CLR as I don't know if it will attack the alloy block either though I tend to think it would be alright.

There are also proprietry cooling system cleaners, some of which must be compatible with alloy engines.
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