Drill/Impact to turn water pump?
 

My situation:
I'm on the finishing stages of reassembling my engine bay after full tear down of everything. The car is a race car and I am doing a megasquirt install so the accessories are at the base minimum.
I am also installing an aluminum radiator.
When I drained the cooling system it was solid orange and cloudy from oxidation in the irons. But when I pulled the rad and water pump etc, there is really a lot of rusty gunk in the system.

I don't want to install my nice new radiator and immediately pump gunk into it so I want to do a flush, but not sure how to accomplish this with the cooling system disassembled? I was thinking then, if I feed water through a bucket or whatever into the water pump and hook a drill or impact gun to the water pump pulley. Could I not flush the system that way? I have a feeling the drill/impact method isn't doable but not sure why. Any thoughts?

Thanks!

yep, should work.

Using a drill to turn the pump should be fine.
I'd make sure to keep the bucket as the high point in the system running both the feed and return line into the bucket.
That way all the air will be out of the system and you'll get a more thorough cleaning.
You could put some filter fabric or fine mesh to act as screen to keep debris from getting sucked back in.
You will also need to pull the thermostat to open up the flow while cold.
Alternatively, you could feed the system using a hose or other pressurized system and return it into a bucket, again keeping the line under water to prevent air from filling the system.

If it was really bad, I'd use a Prestone flushing liquid or similar as they will have engine safe detergents to help break up the build up. Just do a thorough flush with distilled water before filling with whatever mix you normally run.