If you allow 1 stop loss of sensitivity for every 10 years out of date, then I think you'd be looking at ISO6 or below. Better pack a tripod !
ISO 25 then?
TBH, I would do just that - aim it at hipsters on e-bay, possibly with a silly reserve. Just make sure you list it as Kodacolor II rather than the III you've entitled the thread as - some clever Lomoista would complain that it was misdescribed and demand a refund...
I'm going to take it to turkey and shoot it in comedy style at 24 ISO As @Asha said close to wide open
20 shots !
I actually suggested wide open at box speed but either way have fun.....i'd be interested to sée what, if any results come from it.
I would recommend that if you intend shooting any "important" shots, that you duplicate with another film/camera of which you are confident will give you the result you want in case this film goes t*ts up.
I wonder if some mug would pay a huge amount of money for my 3 rolls of Kodachrome 64 expired in 1991? The buyer might be a bit disappointed when they posted one off in its pre-paid processing envelope.