Couldn't get mine out, and mine are allen.

When I put the 3.5 heads onto my 4.5 I used a garden sprayer as a "Pressure washer" to hose down the inside of the block & I used a 1/2" ID hose hooked to a Shop Vac to suck the water out. Worked very well and my engine ran MUCH cooler after this. Now, taking your heads off to flush the block is probably a bit extreme, so do what Ron did & hook up a garden hose until it runs clear!