Mark,

If 12" x 18" or even 16" x 24" is a far as you want to go from 6x9 then you should be able to use one of the 800 speed films with no worries at all. I think that the current version of Portra 800 has a slight edge over NPZ (now called Pro 800Z: same film, higher price) but it is a very close call, and in mixed lighting you might prefer NPZ, but again, it's a close call.

'Overexposure' is your friend in artificial light with daylight-balanced neg film, so set your meter to 400 or probably even lower. This is a good reason, in my opinion at least, to use 800 film instead of 400. Colour neg film has huge overexposure latitude and doesn't get grainy with overexposure. By 'overexposing' you give yourself room to adjust the colour balance, and to keep plenty of detail in dark skin. If you do want to use a 400 speed film, Kodak Ultra 400 is worth considering because of its great latitude and low graininess.

Best,
Helen