I just did this on a 29 Model A motor. When I took the lower water neck off, I found that the block was full of dirt, sticks and (of all things) acorn shells. A squirrel or mouse must have made its home in there in the 40 or so years the A wasn't running.
I ran a garden hose and ran the water forward and backward for a while and poked around with a screwdriver to loosen any junk. There was a lotta stuff that came out, but when I was finished, I couldn't see or feel anything but the cylinders inside. I think I got it all out.
This is probably different from the kind of build up you might have in your motor but as 64Fleetside suggested, you might be able to use something in the same way I used the screwdriver to dislodge as much junk as possible. I'd sure want to make sure that the water wasn't trying to flow thru the radiator to avoid plugging it up.
