Thanks for your input, guys. I have a single 3" conduit, so I did some research in section 230 on service ampacity. As long as the computed load is less than the service ampacity, the cable can have a smaller ampacity than the overcurrent rating of the service. I didn't want to run parallel cables from the 320-amp meter to each panel. It looks like 350-alum will be plenty big for the two panels. It will splice in the old CT cabinet. (The CT's are going away). Thanks again for your replies!