Got the block out of the truck,
Do you mean it is a stripped down block - with the heads, intake manifold and water pump removed?
That requires a hot tank type cleaning. But if you mean that the engine is still intact, you can do a real good flush of the watersides. To really clean the cooling passages, set up a recirc pump and flush the system with a heavy solution of CLR (use the ZEP brand - it's much more effective).
Basically it goes like this:
Get a good-sized bucket, some kind of a small pump, and the necessary pipe fittings/hoses. The pump takes suction from the bucket, tee off the pump discharge and connect to both block drains. At the top - connect a hose to the thermostat housing outlet and lead it back to the bucket. Block off the water pump inlet from the radiator and the heater hose nipple on the intake manifold. Add a bottle of CLR to the bucket. Start the pump and add just enough water to the bucket so you get a flow out of the thermostat housing (without sucking the bucket dry).
Let the pump recirculate the solution for a day or so.
Pull the hoses from cooling system drains on both sides of the block and flush from the top down.
I have always been able to get the plugs out.
Just use a 6-point socket and tap the heads a few times with a hammer before cranking on them.
If they are stubborn, use the old trick of cranking them in the tightening direction first and then try backing them out.
When you pull them be aware that the coolant passages behind them will be absolutely jam packed with sediment. It will take some serious digging just to get a trickle started - and then for quite a while after they will keep getting plugged up again.
But eventually you will get a good solid stream of clear water flowing out.
At that point you can start chemically cleaning with a re-circ pump (described above).
