Anyhow, I acquired a few rolls of Ektarchrome 64 and 320 Tungsten which I plan to cross processing. These rolls have expiry dates of between 2000 to 2005.
Nice experiment. I have no clue to the real answer, but I would just try it at box speed. I shoot Fuji NPH400 and Fuji NPc160 from early 2000's at box speed without problems. There is a slight but beautiful shift in colours.
I've shot very old Kodak Gold but that film turned into muddy brown colours. I wonder how these Ektachrome turn out. Maybe you could develop one normal in C-41 as a reverence?
I'd overexpose slightly -- maybe shoot at 50 and 200. You might also considering pulling the film at say 2:45 to control contrast (given a 3:15 process time). Just some thoughts based on my own experience. I cross-process Provia and Ektachrome pretty regularly and make C prints. I've also cross-processed Fujichrome T64, but I've only scanned it.