Slant 6 Coolant Flush

I have a bit of dilemma. I need to flush the coolant system in the motor but I don't have a radiator to use. The radiator has a hole in it and leaks bad.

I do have a new radiator to install but, I don't want to use it to flush the engine. If I do, I'm going to be polluting this new radiator with all this rust crap.

Any ideas?