It will be a double, with a 9-speed FD. Cassette and shifters are 8 speed. I think I'll just hold off on the new chain temporarily and try to set it up with my existing 8-speed chain. If there is rubbing on the FD, I'll try a 9-speed chain.
My first worry was that the narrower 9-speed chain would get stuck 'between gears' on the 8-speed rear cassette. Is this a legitimate possibility?