Cliff, I'm surprised that your Nissan Parts guy told you that you had to buy 10 of the outer O-rings. The Part number is for one. Maybe things are different in the USA, but in Canada I was able to order one and it came in it's own individual sealed Nissan package.
There is another way to make a seal for to replace the " Top Hat " seal. Clean the inside of the speedometer cable ( Transmission end ) fitting thoroughly with Brake Clean. Then apply some light grease or oil to the inner cable for .about the first inch or two. Then get some Automotive RTV and partially fill the inside of the screw coupling . It doesn't have to be very thick, but you basically want to make an RTV " washer " that seals the cavity and the inner cable. The oil or grease on the inner cable will prevent the RTV from sticking to it. Now screw the speedometer cable coupling assembly back onto the speedo gear housing. Wait about 30 minutes before using, or whatever set time for your RTV is. Voila, you've just made a new Top Hat. Did this to my old Rally Datsun back in the " Day " it worked slick.