OK- gonna take a stab at this one-
Since the F-bodies are so closely related to the M-Bodies...
Over at M-Body forums there is a tech link for upgrading to a 3-core aluminum radiator designed for the 87 Dodge Ram/Ramcharger.
I used this radiator in my 82 NYer....and authored some of this link.
Here's the link-
http://www.dippy.org/forum2/index.php?topic=24.0

You MAY be able to use this radiator in your RR. Maybe, Maybe not, but it's a cheap alternative.

Hope this helps,
CC.