This isn't a direct answer really, but something to think about.These days the "average" consumer film scanner can give you a file at least 24 MB
in size. Most can go well above this.
If there is a full service camera shop near you, I would think you can have them do it, or they can send it out for a drum scan or Kodak Pro-CD scan which will give you about 90MB
In that there seems to be some knowledge of the technical aspects of this on the customer's part, I'm surprised he didn't specify the desired DPI. Also likely you may hear, "I can't open the file" or "my printers say their machine can't read the file." Having a camera shop print (reproduce in some way) the poster and mailing it to the customer would avoid this.
Of course Kodachrome 25 would be preferred for a poster, but if K64 is what you have, so be it. Keep in mind that if you supply the image on CD
, you are leaving the possibility of unauthorized image reproduction in the hands of the customer.