Fantastic Vent makes two different trim pieces for early and late model Airstreams, and, if I recall correctly, in your choice of ivory or white. It may very well be true that the new wide bodies are near enough flat at the center as to obviate the need for curved trim.
Give them a call and they will send you the correct trim for your coach at no charge.
The difficulty will depend largely upon your proximity to a 12v
electrical source to run the fan. Usually people tap off a nearby light fixture. Do the 61's run a major wiring run just off the center of the roof line, as do the later models? Whether or no, you will need to be very careful about drilling into electrical wiring. Expect to have a great deal of difficulty removing the old vent. 42 year old sealant can be very tenatious.
I would install with rivets, rather than screws. You can use regular aluminum pop rivets, but use plenty of Vulkem to seal them.