You could certainly hoist a 420 spinnaker on a Catalina 16.5, but it might be too small to function well. For a spinnaker to work well the spinnaker pole needs to be equal to "J" - the length from the mast to the forestay tack fitting. The 420 pole length is 1.75m, and your J length is 1.7m, so that should be OK. The height of your spinnaker hoist - "I" - is the problem. The 420's hoist is a touch under 14'. The hoist on your Catalina is almost 19'. The 420 sail will be about five feet too short -- the spinnaker pole might not even be able to reach the clews. Having it up so high might also tend to make the boat heel over more than pull it forward. You need a sail from a boat with a taller mast than the 420.