Skin effect is huge as the conductors get huge. Unless spec'd the larger conductors are uneconomic.
Paralleling a fist full of 1/0s will give you maximum un-derated capacity.
The rules exist based upon heat buildup/ radiation. In underground/piped systems there is a limit as to how fast the heat can bleed off. While aboveground conduit would seem to have advantage -- remember that air temperatures of 100 F are possible. This means that worst case calculations show either route rate limited.
Since these conductors are crossing the border -- I'd hand the issue off to an EE and get him to take the exposure.
I can't see anyone paying an EC for his professional expertise in such matters. Think of yourself as a general practitioner and the EE as a surgeon. No one wants your opinion on heard operations. They certainly aren't going to pay for it.
Concentrate on what you can get paid for -- not on what piques your professional interest. Keep that as a hobby -- and don't expose yourself to professional liability.
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I've had instances where I was right and the 'expert' was wrong. It went his way all the way down the line. I had no business being that knowledgeable. My boss was not interested in defending my position in the slightest.
Being correct -- but beyond your credentials -- will get you killed, professionally.
