so the inspector finally approved the wiring method after submitting a load calc to him. the load calc came to 359amps and 125 on the neutral. He is now telling me to install a meter rated at or above 359 he specifically said not the 320 common meter. A "400" amp meter base is not manufactured. The base a submitted is rated at 320 continuous/ 400 max although we know the load calc is not continuous will this suffice.
Could your inspector be overstepping his area of jurisdiction? He is basically saying you need CT metering on this house. Working for the POCO, I once did a 10,000+ sq ft house with CT metering. But we did quite a few million dollar houses with 400 and 600 amp services, always with your basic self contained 320 amp meter enclosure (rated, as you rightly note, 320 continuous and 400 peak).
Hell, you could likely have left the existing 4/0 aluminum alone and that would have fed the upgraded service fine. Admittedly I don't know all the particulars so maybe not but it wouldn't surprise me.
I might speak with the POCO and get their take. I can just about guarantee you that have no interest in putting CT metering on a house whose load will never justify it.