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SteveKen did a lot of research on this stuff in his S6-RS6 conversion: http://www.rs6.com/showthread.php/25661-S6-conversion-to-RS6?p=259812&viewfull=1#post259812

Steve, did you ultimately remove...

I wonder if you could replace that third fitting with a tee off the lower radiator hose. I know it's a bit kludgy, but with the huge price difference between the regular 4.2 radiator and the RS6...