Interesting disscussion.
What about a bit differant approach.
Set up the pump, res and rad in a external case of their own. This would allow you to design for a Xtreme sized rad. I'd suggest looking at rad #2-192 in the heater core data base.
10.75" X 10.25" X 2" for the finned area. A huge core that can handle about anything I can imagine a single system having, even TEC's on CPU & GPU. And no messy cutting of tanks, just shorten the odd inlet & outlet tubes.
And this large of a rad would greatly reduce air flow resistence form your dual 120mm fans. Or perhaps some really large fans, 172mm push=>pull anyone? Small squirrel cage blower of 8-10" tall x 6-8" wide?
And as long as you are going with a external case for max attempt at reaching ambient temps you will have room for a pump of any size you desire (Little Giant or Iwaki?). And the above rad has 5/8" inlet & outlets so you could also increase your line size to 5/8" as well for lower flow resistence in the lines.
This could become a never ending spiral, eh?
But if anything is going to come very close to ambient, this monster sure should!
I intend to build something along these lines for my new system. The exterior cooling case will be elongated and open at both ends as a sort of wind tunnel. And with the cooling case elongated the puter case will sit on top of it. My water lines will go up through the puter case's floor. Rad will be 2-342 however and pump a Iwaki MD-15 & DIY custom res. IN my build one goal will be to maintain the lowest noise level I can.