If I knew that the block had gone through the manual cleaning process, and
that the radiator was clean, I would flush the system one more time, then  
add a can of water pump lube/cooling system conditioner to the 50-50 mix.
It will look milky at first, but will clear up in a few days and you
shouldn't see the rusty tint again.

Anyone ever tried any detergents or other cleansers in their cooling
system?  I've put my '55 back together and the cooling system seems to
be mostly clean but the radiator fills up with a fine coating of rusty
goo.  I suspect that this is surface rust and residual gunk left over
from cleaning out the block, and appears that it would "wash" off with a
mild detergent, but what to use?  I'm half tempted to use some automatic
dishwasher detergent (because it doesn't make as many suds as the normal
stuff) and drive around with that in there to see if it does the trick.

Of course this will have to wait as I still haven't gotten the fan
shroud and air deflector (or whatever you call the plate that holds the
hood latch) ready for paint yet...  a pox on the previous owner of the
car; you can almost measure the thickness of the old paint with a ruler.
 I'm guessing getting stuck in traffic, or even idling in the driveway,
with no fan shroud would be an unpleasant experience.

nate