Hi -
Nice car - my grandpa had the sedan, same vintage. Nice car to tool around in.
But that's a tall order, doubling the fuel economy. Sounds like you don't have a baseline yet, so that would be the first step.
The car probably has a fuel economy computer. A ScanGauge will give you more to work with and will help most when it comes to refining your driving, which is the quickest path to the biggest savings right off the bat.
As for mods, have you seen our list?
65+ Vehicle modifications for better fuel economy - EcoModder.com