Like many others that bought military surplus steel NATO fuel cans , I was frustrated by the fact that only large diameter leaded fuel spouts were available for these cans. This meant that I had to either use a funnel or transfer the fuel to another can before putting it into the tank of a car with an unleaded (small diameter) fuel receptacle. A friendly employee at my local Lowe’s found me an inexpensive solution. I brought my spigot to the store so I can test fit items (always a good idea for any home improvement project), and he gave me the following two items:
1.) A Genova 1″ x 3/4″ Polypropylene Coupling 
2.) A Murray #12 Adjustable [Aero-Seal style] Hose Clamp
I stuck the reducer in the hose that was attached to my pour spout, slipped the clamp over it, and tightened it. I used it tonight with great success!
It cost me $2.74 for two reducers and two clamps, so it was a $1.37 solution for each of my nozzles. – Lee H.