To replace the block because it's dirty doesnt make much sense and isn't the least expensive fix. I would do as others suggested and get it power flushed. If that doesn't do it and you get to the point where you're thinking of pulling the motor, remove the heads, water pump and freeze plugs and give it good washing.
I have had the sand/sludge in one engine and had to pull the freeze plugs to get it out. Assuming you have a 302 do you have a fan shroud because when I didn't if would allow the air to recirculate in the engine compartment and not pull fresh air through the radiator.
